Every page is Markdown now, so drop `.rst` from `source_suffix` and clean up the
`.rst` references left in the configuration, in `mach doc` itself, and in the
documentation about the documentation.
`eval-rst` blocks are unaffected: they are a MyST feature, independent of
`source_suffix`.
Differential Revision: https://phabricator.services.mozilla.com/D321835
Sylvestre Ledru
e709bfa957