Docs: Write initial Addons documentation page

[ericholscher]

The idea here is probably for @humitos to write an initial draft and work with @plaindocs as a primary editor.

[plaindocs]

OK, @humitos and I chatted about this and reckon the following is a decent approach:

• Find a home for each separate addon within the most relevant section of the existing docs structure (more details on that below)

  • Focus on docs for maintainers (readers shouldn't really need them)
  • Leave advanced js/json config for last

• Add a new Addons page or section listing+linking them with a short description (kinda like the existing marketing page)

  • Add a config section around enabling the addons, or where they are on by default

For homes for addons, I'm thinking something like

• Analytics -> hosting documentation
• DocDiff -> Build process (PR previews)
• PR notification -> Build process (PR previews)
• Version flyout -> Reading documentation
• non-stable notification (have we thought about merging the three banner sections in the docs) -> reading documentation
• latest notification -> reading documentation
• search as you type -> reading documentation (cross linked from search in hosting docs)
• Sponsorship -> reading docs

And while I'm there, doing some refactoring of existing related pages that are replaced by the new addons (being phased out), including

• https://docs.readthedocs.io/en/stable/flyout-menu.html
• https://docs.readthedocs.io/en/stable/versions.html#version-warning
• https://docs.readthedocs.io/en/stable/server-side-search/index.html

I'll get started on this early next week