Improve links in the usage/tcja_after_2025.md document

[martinholmer]

Update hypertext links to show GitHub pages for the ext.json reform file and for the extend_tcja.py script.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.42%. Comparing base (ea93c7a) to head (06c1d0c).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2735   +/-   ##
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  Coverage   99.42%   99.42%           
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  Files          13       13           
  Lines        2594     2594           
=======================================
  Hits         2579     2579           
  Misses         15       15           

Flag	Coverage Δ
unittests	99.42% <ø> (ø)

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