doc/badges.py: Adding staticly generated HTML page for automated redirects.

In addition to publishing the latest badges with every commit to master,
we now also generate a release.html and snapshot.html file which is simply
a redirect to the latest release and snapshot url on github.

This allows us better control of the URL which the badges link to,
now the README.rst redirects the latest release/snapshot when clicking
on the corresponding badges automatically with the following snippet:

  .. image:: https://docs.buildstream.build/master/_static/release.svg
     :target: https://docs.buildstream.build/master/_static/release.html

  .. image:: https://docs.buildstream.build/master/_static/snapshot.svg
     :target: https://docs.buildstream.build/master/_static/snapshot.html

Changes:

  - README.rst: Use the new link targets for the release/snapshot badges
  - doc/badges.py: Support generating the redirect html files
  - doc/Makefile: Generate the redirect html files during the build
3 files changed