Update index.rst to explicitly link to the mozilla-l10n sites (#3078)

Per discussion in #2135 (comment), make it clearer that there are actually two different external subsites containing pontoon documentation. 

Relates to #2214

---------

Co-authored-by: Matjaž Horvat <matjaz.horvat@gmail.com>

docs/index.rst

@@ -16,6 +16,17 @@ them back periodically. Typically these external sources are version control
 repositories that store the strings for an application. Supported external
 sources include **Git**, **Mercurial** and **Subversion**.
 
+Other Documentation
+-------------------
+
+Important additional Pontoon documentation can also be found at the following sites:
+
+`How to localize with Pontoon <https://mozilla-l10n.github.io/localizer-documentation/tools/pontoon/>`_ (Mozilla Localizer Documentation)
+
+`How to manage localizations on Pontoon <https://mozilla-l10n.github.io/documentation/tools/pontoon>`_ (Mozilla Internal Tools Documentation)
+
+(For an issue to improve the documentation, see  `#2214 <https://github.com/mozilla/pontoon/issues/2214>`_.)
+
 Contributing
 ------------