Enable Pages / github.io settings for aqa-tests repository

Now that the documentation is organized to be served as a Github.io page, we can enable the settings on the aqa-tests repo to serve it as a web page.

We may also want to reformat the 4 category index pages as part of this enablement:
- https://github.com/adoptium/aqa-tests/blob/master/docs/pages/aqa-explanation.markdown
- https://github.com/adoptium/aqa-tests/blob/master/docs/pages/aqa-howto.markdown
- https://github.com/adoptium/aqa-tests/blob/master/docs/pages/aqa-reference.markdown
- https://github.com/adoptium/aqa-tests/blob/master/docs/pages/aqa-tutorials.markdown