Fix up the 'Templates' section

[kilfoyle]

This attempts to improve the Templates docs by:

• Adding more explanation into the page rather than just linking to the API docs.
• Removing the "manage index templates" content that is duplicated; this section in the more prominent "Index basics" will be the single source of truth.
• Improving the narrative by making a clearer distinction between "Index templates" and "component templates", with separate sections for each.
• Changing the giant warning about index pattern collisions (which takes about 1/3 of the page) into a subsection of "Index templates"; this should flow better and be less jarring for readers.
• Including the API instructions for creating each kind of template, and also link to the Kibana version of those steps that are included in the Index basics page.
• Adding a tip with links to the subpages about simulating an index template and about ignore missing component templates, just to help people be aware of these cool features.

Please see preview page: Templates

@eedugon This was prompted by your suggestions here so I hope you don't mind my adding you as a reviewer. 🙏

Closes: #2056

[github-actions]

🔍 Preview links for changed docs

• manage-data/data-store/index-basics.md
• manage-data/data-store/templates.md
• manage-data/data-store/templates/index-template-management.md

[eedugon]

Looks great! I've added some suggestions for the explanations and descriptions around component templates and index templates.

btw, I've also found this insteresting blog that I'm not sure if it could make sense to link also: https://www.elastic.co/search-labs/blog/index-composable-templates

In this doc we also mention legacy templates but we don't give any context or link..... so I don't know how best to address this. I don't even know if we have the old legacy templates behavior documented (this is an old doc about legacy templates).

[eedugon]

I feel the word tools a bit vague in this context, and I'd like to treat templates in singular, as the mechanism. Let me know your thoughts about this proposal:

Suggested change

	Templates are tools used to construct indices in {{es}}. They specify the properties that each created index will have.
	Templates are the mechanism that {{es}} uses to apply settings, mappings, and other configurations when creating indices or data streams.

(I like including also data streams, as the way to create data streams is also through templates).

Another possibility:

Templates are the mechanism by which {{es}} applies settings, mappings, and other configurations when creating indices or data streams.

[eedugon]

Suggested change

	You configure templates prior to creating indices. This way, when each index is created, either manually or through the indexing of a document, the template settings are used as a basis for creating the index. When used with a [data stream](/manage-data/data-store/data-streams.md), a template configures the stream's backing indices as they are created.
	You configure templates prior to creating indices or data streams. When an index is created, either manually or by indexing a document, the matching template determines the settings, mappings, and other configurations to apply. When used with a [data stream](/manage-data/data-store/data-streams.md), a template also defines how each backing index is configured as it is created.

[eedugon]

Suggested change

* An [**index template**](#index-templates) is used to configure indices when they are created, by specifying the mappings, settings, and aliases that each index inherits.

* An [**index template**](#index-templates) is the main configuration object applied when creating an index or data stream. It matches index names using `index_patterns` and resolves conflicts using a `priority` value. An index template can optionally define settings, mappings, and aliases directly, and refer to a list of component templates that provide reusable configuration blocks. It can also indicate whether it should create a data stream or a regular index.

Not sure.... maybe too long, but in the intro to index templates I think there's something important to highlight:

• is the component that has to match the index or data stream name
• it's used to determine if the item to create is a regular index or a data stream (for this we can just link to data streams documentation).
• can optionally include settings, mappings, and aliases, and refer to component templates.

The current description just puts the focus on mappings, settings and aliases, which is more aligned with the legacy templates and old way of working, without using composable templates.

[eedugon]

Maybe you prefer something much shorter here and describe the points I mention later in the section dedicated to index templates. Whatever you prefer :)

[eedugon]

Suggested change

	* A [**component template**](#component-templates) is a reusable building block used to construct index templates.
	* A [**component template**](#component-templates) is a reusable building block that defines settings, mappings, and aliases. Component templates are not applied directly; they must be referenced by index templates.

[eedugon]

Used to construct index templates sound a bit weird when we want to mean that index templates include component templates to reuse their contents.

Suggested change

	A **component template** is a reusable building block, containing mappings, settings, and aliases, used to construct an index template. Component templates are used only to construct index templates and are not applied to indices directly.
	A **component template** is a reusable building block that defines mappings, settings, and aliases. Component templates are not applied directly to indices, but referenced by index templates and used when determining the final configuration of an index.

[eedugon]

Can we add a link in legacy templates? The concept of legacy templates doesn't appear in the doc, and might be difficult for a reader to understand what's about.

[eedugon]

Adding one sentence that can be the glue for what's coming next.

Suggested change

	The following conditions apply to using templates:
	Together, index templates and their referenced component templates form what is known as *composable templates*.
	The following conditions apply to using templates: