Move gh-pages branch out of hpx repo

[msimberg]

We've collected quite a bit of bloat in the git repo due to the gh-pages branch. About 90% of the size of the repo comes from gh-pages commits (170 MB cloned without gh-pages, about 1.7 GB with it).

I suggest we start a new repo hpx-docs which will get the automatically built docs, and we delete the existing gh-pages branch (except for the last commit so that we don't break links, plus possibly some appropriate forwarding pages).

Alternatively, if we don't care about history (we can after all in principle recreate the gh-pages history from this repo) we can just force push a single commit to the gh-pages branch. This avoids breaking any links.

Any preferences?

We could actually get the repo down to way under 100 MB if we rewrite history. There are some old commits with some sort of generated source files and at least one rar archive (over 100 MB). I'd only do this as a last resort though as it forces everyone to reset their local branches (maybe for HPX 2?).

[hkaiser]

Doesn't GitHub.io pull everything from this branch?

[sithhell]

We can tell the GitHub page generator to look elsewhere. I support this move as well as rewriting history.

[msimberg]

The GitHub pages are repo specific, so we'd end up with docs under stellar-group.github.io/hpx-docs instead of stellar-group.github.io/hpx. We can still keep an almost empty gh-pages branch here which redirects to hpx-docs. The second alternative doesn't change any URLs.

[msimberg]

I've now moved the documentation to the hpx-docs repo. It still has the full history that we had here on the gh-pages branch. Here I've kept one commit on the gh-pages branch to keep links working. I'm closing this now but please reopen if you find problems with broken links etc.