Link to the change log in release notes

[RazerM]

Release bodies render as Markdown on GitHub, but the notes were extracted verbatim from the RST change log, so 2.0.0 published with raw RST markup.

Replace the extraction with a link to the tagged Read the Docs build of the change log. This drops the awk parser and the checkout that only fed it; GH_REPO gives gh its repo context instead.

The 2.0.0 release body has already been updated by hand to match.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.31%. Comparing base (8258f51) to head (64837fb).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #174   +/-   ##
=======================================
  Coverage   98.31%   98.31%           
=======================================
  Files           7        7           
  Lines        1308     1308           
  Branches      135      135           
=======================================
  Hits         1286     1286           
  Misses         12       12           
  Partials       10       10           

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