Include user & developer documentation into the BuddyPress repository

[imath]

Now we’ve migrated to the BP Rewrites Parser, we need to achieve an important documentation work.

We also need to renovate our documentation plugins and themes, if you look at ​https://meta.trac.wordpress.org/changeset/12056 You’ll see the wporg-developer theme is no more included into Meta Trac and is moving to a Block theme . As our bporg-developer theme is a child of the wporg one, we’ll face an issue sooner or later as to my knowledge it’s not possible to build a block child theme.

I think we should care about documentation as we care about code and reward it as we do for code, using props so that contributors can get a beautiful BuddyPress badge on their WordPress profile. That’s why I believe we should include a new docs directory inside the BuddyPress repository and enjoy the Markdown importer the handbook plugin is making available. We could use GH PRs to manage documentation updates and synchronize the BuddyPress documentation sites (developer and a new user one) with our GH repository.

Having our documentation into the BuddyPress repository would also allow us to wait for the plugins/theme renovation work to be achieved.

I've tested a quick implementation building ​this plugin. Below is a screenshot of the result on my local development environment (VVV + Meta).

As you can see the content of this PR's /docs/developer/README.md has been added to my local BuddyPress developer documentation site.

Trac ticket: ​https://buddypress.trac.wordpress.org/ticket/8897

[imath]

In 13501:

Add a /docs directory to provide User and Developer documentation

Just like the WordPress REST API or Gutenberg did, we are now using our GitHub repository /docs directory to contribute to User and Developer documentation.

This commit inaugurate this new space with some documentation pages and assets.

Fixes #8897
Closes ​https://github.com/buddypress/buddypress/pull/102