Support Index generation (Glossary)

[garvinhicking]

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Feature request

Sphinx allows to create indexes like this:

https://docs.typo3.org/m/typo3/reference-coreapi/12.4/en-us/genindex.html

This interprets RST tagged indices:

.. index:: 
   XLIFF; Files
   File; XLIFF

and with that information creates a reference index as "genindex.html".

Having an index like this helps readers of documentation to easily see which terms have been referenced on which page, where manual indexes have been placed.