docs: expand trchain runblock examples

[Tobias-Fischer]

Summary

• expand the transform-chain documentation with runnable 3D and planar examples
• demonstrate every transform token, joint and named variables, expressions, built-in and user-supplied functions, angle units, and custom joint prefixes
• show parsed-token reuse and symbolic evaluation

Validation

• Sphinx HTML build succeeded
• all five new runblocks executed and rendered with their output
• git diff --check passed

The build retains existing unrelated warnings from optional dependencies and older ETS docstring examples; the new trchain runblocks emit no warnings or errors.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (d07a90c) to head (eb9da0b).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #655   +/-   ##
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  Coverage   0.00%   0.00%           
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  Files        143     143           
  Lines      14050   14050           
=====================================
  Misses     14050   14050           

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