The favicon on restic.readthedocs.io still contained the old
superman-style logo.
This changes it to the 32x32 gopher icon also used on https://forum.restic.net/ ,
just converted to .ico using Gimp.
Tested by building the documentation and opening index.html in Chrome.
The new favicon looks fine.
* convert documentation to reStructuredText
* switch from mkdocs to sphinx
* rewrite README.md (now .rst) to include less information and be more
attractive