Use permalinks in the changelog

[kmcginnes]

Description

Replace relative doc links in the changelog with GitHub permalinks pinned to the correct release tag. The changelog should be a snapshot of each release, so links should point to the files as they existed at that point in time rather than following main.

• Update ./README.md link in v2.1 to permalink at v2.1.0 tag
• Update ./additionaldocs/features/README.md and ./README.md links in v2.0 to permalinks at v2.0.0 tag
• Update additionaldocs/sagemaker/ links in v1.9.0 to permalinks at v1.9.0 tag

Validation

• Verified each link resolves to the correct file on GitHub

Related Issues

N/A

Check List

• I confirm that my contribution is made under the terms of the Apache 2.0 license.
• I have run pnpm checks to ensure code compiles and meets standards.
• I have run pnpm test to check if all tests are passing.
• I have covered new added functionality with unit tests if necessary.
• I have added an entry in the Changelog.md.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.86%. Comparing base (c5affc5) to head (f8c02bf).
⚠️ Report is 93 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1569       +/-   ##
===========================================
+ Coverage   47.81%   64.86%   +17.05%     
===========================================
  Files         382      372       -10     
  Lines        8525     8382      -143     
  Branches     3159     3123       -36     
===========================================
+ Hits         4076     5437     +1361     
+ Misses       3070     2088      -982     
+ Partials     1379      857      -522     

Flag	Coverage Δ
unittests	64.86% <ø> (+17.05%)	⬆️

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