[clang-doc] Index pages for all instances of a construct

clang-doc currently doesn't keep reference to all the instances of a construct, like a list of all the functions or classes. [hdoc](https://docs.hdoc.io/hdoc/llvm-project/functions.html) and [Doxygen](https://clang.llvm.org/doxygen/annotated.html) have functionality for this. This should probably be done during JSON generation when Infos have already been reduced.

Somewhat related: Doxygen also keeps its own sort of source tree with filenames organized by namespace. This is actually possible (I think) currently using the CDCtx Index.

Start with clang-doc's JSON generation stage, where the issue says Infos have been reduced, and review the CDCtx Index mentioned as related context. Compare the desired function and class index pages with the hdoc and Doxygen examples. Done means generated documentation includes index pages covering all instances of each construct.

Written by the indexing model from the issue text.