Improve linkage of CMake documentation

[thomas-bc]

Originating Project/Creator
Affected Component	docs
Affected Architectures(s)
Related Issue(s)	#1912
Has Unit Tests (y/n)
Builds Without Errors (y/n)
Unit Tests Pass (y/n)
Documentation Included (y/n)

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Change Description

Fixes #1912

Rationale

Better linkage of the exhaustive CMake docs we have. Also fixing some out-of-date stuff

[thomas-bc]

I have made some additional changes to improve linkage of documentation overall.

• added index_gen.py: takes as input a directory and outputs an index of all the markdown files nested within
• using that to generate the CMake index
• added a run of that script at the top level of the documentation -> generates a full index of the entire documentation. Maybe it could be linked at the bottom of the side-bar table of contents?

[LeStarch]

Ready for merge when you are!