Move gh-pages to master

This PR, and its companion PR concerning the gh-pages branch, move the site sources from the entirely separate `gh-pages` branch into a sub-directory `site/` in master branch.

This way, changes can be made to code and to the site sources at the same time. To update the site, we do a "subtree merge" from master into gh-pages and push that. These steps can happen independently to merging PRs and so on.

The subtree merge is done in a script on the gh-pages branch, introduced in the companion PR.