Long-term purpose of this MR is to allow shipping offline HTML
documentation along with our releases, so you can always locally access
HTML docs for your specific installed version (online documentation is
only for latest version). This is a lot more user-friendly than manually
browsing an unorganised bunch of Markdown files.

Simplified logic of which files to include for `mkdocs`: everything in
the `docs` folder plus any `.md` files outside it. This dramatically
reduces the run-time of `mkdocs` and the size of the generated
documentation folder. It also broke links to source code files. These
links can be removed in a follow-up MR.

Additionally:

* Changed site directory from `site` (default) to `doc-html`.
* Renamed `files-for-mkdocs` directory to `doc-html-src`.
* Moved `mkdocs.yml` into `doc` folder.

This also makes the following changes for locally building
documentation, but these changes are reversed when building online
documentation in Gitlab CI:

* `doc/release-notes.md` is now included, as "current release notes" (at
a version tag, this file contains the actual release notes for that
release; in between version tags, the file contains changes to master
since last release).
* Directory URLs are turned off (do not work locally).
* Search plug-in is turned off (does not work locally).

Implementation of the ninja target is loosely inspired by `ninja
translate`.

Test plan: `ninja doc-html` and browse locally from
`doc-html/index.html`, and also check
https://bigblockiftrue.gitlab.io/bitcoin-cash-node/