Update index.rst to explicitly link to the mozilla-l10n sites

[srl295]

Per discussion in https://github.com/mozilla/pontoon/issues/2135#issuecomment-1910356465, make it clearer that there are actually two different external subsites containing pontoon documentation. This wasn't clear to me from the docs.

Relates to #2214

[mathjazz]

Hey @srl295, please let us know if you'd like to make the requested changes. I'm happy to apply them myself.