Consolidate high level Drupal core documentation in markdown files and publish them via gitlab pages

Problem/Motivation

In #3463660: [policy, no patch] Document high-level API concepts in an easier format we decided to consolidate high level documentation into markdown files, and then publish them via gitlab pages. Opening this to plan the concrete steps to do that.

Steps to reproduce

Proposed resolution

Initially I think we should do something along these lines, better plans are welcome:

1. Convert docs for one subystem to markdown. In #3463660: [policy, no patch] Document high-level API concepts in an easier format I used the example of the cache API because I'm familiar with it and the docs are very much split between api.drupal.org and the handbook, but we could pick another one. Issue for this at #3582253: Markdown docs for cache API

2. Set up gitlab pages for core on gitlab pages to publish the files in that directory. This should give us a single page that's published then.

3. Review and fix the format/structure of that page so that we're reasonably happy with it.

4. Open issues to do other subsystems.

As we're doing this, we should also remove the .api.php high level documentation, and replace the handbook pages with links or redirects to the new markdown documentation, so that we go from documentation in two places to having it in on place, instead of three. But that should probably be part of step 4

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet