Docs: Parameter schema

[ewels]

We could really do with some long-form documentation about nf-core parameter schema files.

At the top we can write DO NOT EDIT MANUALLY, USE "nf-core schema build" which will help a lot of people.

After that, should talk about:

• General purpose
• Function within pipelines
• Function with nf-core launch
• How to use nf-core schema build (from scratch and to update)
• API spec of the different JSON schema fields

[apeltzer]

• including the docs building that was recently added.

[jenmuell]

PR created for nf-core schema build (from scratch and to update) and for the nf-core schema docs command nf-core/tools#1926

[jenmuell]

• General purpose and function in pipelines are included at the beginning of the Pipeline schema section
• Function with nf-core launch is mentioned in the Launch a pipeline section

[FranBonath]

can this issue be closed?