Commit 4ae41f9c63 made `git_odb`
race-free, and added internal locking. Update `docs/threading.md`
accordingly, so that APIs built atop libgit2 (e.g. language bindings)
can count on this.
Our non-technical documents are currently floating around loosely in our
project's root, making it harden than necessary to discover what one is
searching for. We do have a "docs/" directory, though, which serves
exactly that purpose of hosting documentation.
Move our non-technical documentation into the "docs/" directory. Adjust
all links to these documents.