in #6083 the test runner was renamed to libgit2_tests,
but not all references to the old name were updated.
this change changes all of them to use the new name.

Update from regex to pcre licensing information in docs/contributing.md

The name of libgit2's main branch is now "main" but contributing.md and
release.md still reference "master" as the main branch.  Fix it.

While our contribution guide tries to make clear the licenses that apply
to libgit2, it does not make clear that different licenses apply to our
bundled dependencies. Make this clear by listing each dependency
together with the licenses that they are governed by. Furthermore,
bundle the complete license texts next to the code they apply to.

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.