Include Github as a download mirror

Currently there are only 2 download site for boost. One is jfrog which is still [unavailable](https://github.com/boostorg/boost/issues/842) at the time of current writing. Another one is sourceforge which is slow and I had faced reliability issues in the past.
I propose to utilize Github as another (backup) download site.

Boost is already putting release artifacts on github: https://github.com/boostorg/boost/releases
The script for creating those artifacts is here: https://github.com/boostorg/boost/blob/master/.github/workflows/release.yml
However those artifacts on github are not the same as from jfrog or sourceforge, they have different file tree layout and different hashes. Please consider fixing them or upload the official ones.

ps. nowadays Github Actions has non trivial amount of CI jobs running (for various projects) and those CI jobs would benefit (in download speed and reliability) if boost can be fetched over the same github domain.