Add support for writing conditional documentation on the docs site

[emmatown]

This will allow us to remove the website_live branch while still allowing us to document new features that will only be available in the next release. The conditionals are removed when doing a release. I was planning on removing them by transforming Markdoc's AST but Markdoc's formatter is experimental and seems to break on some of our content so this uses a regex. I was also planning to add conditionals around documentation on unreleased features but it seems as though there isn't any.

New

{% if $nextRelease %}
You can filter a post using `post (...)`
{% /if %}

Updates

{% if $nextRelease %}
You can filter a post using `post (...)`
{% else /%}
You cannot filter posts.
{% /if %}

Removals

{% if $nextRelease %}
{% else /%}
You cannot filter posts.
{% /if %}

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Name	Status	Preview	Updated
keystone-next-docs	✅ Ready (Inspect)	Visit Preview	Sep 26, 2022 at 5:51AM (UTC)

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