docs(toolkit-lib): ensure all message data interfaces are documented and other improvements

[mrgrain]

Main change here is to fail the build when the docs generation has warnings. This enforces that all referenced message data interfaces are exported and documented.

Made some organizational changes and how the message registry is presented.
Also added basic support for links to external interface from cxapi.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.13%. Comparing base (c71caf0) to head (3cfe69e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #210      +/-   ##
==========================================
- Coverage   85.18%   85.13%   -0.06%     
==========================================
  Files         208      208              
  Lines       35710    35710              
  Branches     4665     4639      -26     
==========================================
- Hits        30421    30402      -19     
- Misses       5132     5157      +25     
+ Partials      157      151       -6     

Flag	Coverage Δ
suite.unit	85.13% <ø> (-0.06%)	⬇️

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