The libgit2 project mostly follows the coding style of git and thus
the linux project. While those two projects use a recommended tab width
of eight spaces, we instruct users to set their editor's tab width to
four spaces. Fix this to say eight instead.

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.