Build the API documentation as part of our own build.

• Allow building the API documentation as part of gst-docs.
• Replace Makefile with a proper meson based build definition
• Generates the documentation as a CI job.
• Adds a way to make a "release" of prebuilt documentation with all the documentation with the release target
• Adds support for devhelp (part of the release tarball)

You can see the result here: https://thiblahute.pages.freedesktop.org/gst-docs/ https://people.freedesktop.org/~tsaunier/documentation/?gi-language=c

NOTE: This depends on the hotdoc branch in all GStreamer module providing API or plugins.

NOTE1: Depends on hotdoc master, but Mathieu is going to make a release RSN.

Fixes #23 (closed)

cc @meh @tpm