Add synapse.tools.pkgs.gendocs (SYN-8828)

[vEpiphyte]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 97.95918% with 3 lines in your changes missing coverage. Please review.

Project coverage is 97.32%. Comparing base (d631ab5) to head (58e360e).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
synapse/tools/pkgs/gendocs.py	97.16%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4331      +/-   ##
==========================================
- Coverage   97.39%   97.32%   -0.07%     
==========================================
  Files         262      264       +2     
  Lines       60152    60299     +147     
==========================================
+ Hits        58584    58685     +101     
- Misses       1568     1614      +46     

Flag	Coverage Δ
linux	97.32% <97.95%> (-0.01%)	⬇️
linux_replay	?

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[vEpiphyte]

Per discussion rename the core tool here to synapse.tools.pkgs.gen_docs.