Restructure GDK documentation
What does this Merge Request do?
Moves core documentation into the doc directory, and non-core documentation out into doc/howto.
Renames some files to index.md, so that the project has only one true README.md file and eventually each directory has an index.md to land on (not the case yet).
I programmatically checked the internal links, so was able to check the links are working and fixed some legacy broken anchors.
In a few places, provided a little polish.
Relates to: !1280 (merged).
Edited by 🤖 GitLab Bot 🤖