Utilities/Sphinx: Fix html_favicon configuration

The value must be either a full path or relative to the configuration
directory, not relative to the 'static' directory.  Use a full path.
This avoids a warning:

 WARNING: favicon file 'cmake-favicon.ico' does not exist

It worked before because all 'static' directory content is copied to the
'_static' directory of html output anyway.
Merge topic 'doc-fix-html-favicon' into next

ca83c39c Utilities/Sphinx: Fix html_favicon configuration