Documentation overhaul

[TheBB]

See #3604 for backstory.

What this PR does:

• Changes all links from org format to github format. This means that reading the docs on github is useless.
• Leaves toc-org enabled on the assumption that it is still useful for reading in Emacs. This may not be the case (org has its own facilities for navigating large files).
• Adds a file called core-documentation.el which is not loaded by default on the assumption that this is something that very few people will do and not very often. This file contains:
  • a function spacemacs//generate-layers-file which generates a file layers/LAYERS.org with a listing of all the layers.
  • a function spacemacs/publish-doc which exports all the relevant documentation to doc/export/ in the form of HTML files. This function automatically calls spacemacs//generate-layers-file.

The contents of the export directory should then be sent to the gh-pages branch somehow. This is not automatic yet.

I might make a Makefile target for this in the future.

[syl20bnr]

Thank you ! Beautiful PR ❤️
Cherry-picked into develop branch, you can safely delete your branch.