Add navigation back-links to all documentation leaf pages

[kmcginnes]

Description

Added "← Back to [Section]" links at the top of every leaf page in docs/guides/ (7 pages) and docs/references/ (5 pages). This helps users who land on a leaf page from a search engine discover sibling pages and navigate the docs structure.

Validation

All links verified by inspection. No content changes.

Related Issues

• Fixes Add navigation back-links to documentation leaf pages #1696

Check List

• I confirm that my contribution is made under the terms of the Apache 2.0 license.
• I have verified pnpm checks passes with no errors.
• I have verified pnpm test passes with no failures.
• I have covered new added functionality with unit tests if necessary.
• I have updated documentation if necessary.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.69%. Comparing base (c5affc5) to head (f0b8a2b).
⚠️ Report is 143 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1701       +/-   ##
===========================================
+ Coverage   47.81%   72.69%   +24.88%     
===========================================
  Files         382      408       +26     
  Lines        8525    12286     +3761     
  Branches     3159     3525      +366     
===========================================
+ Hits         4076     8931     +4855     
+ Misses       3070     2337      -733     
+ Partials     1379     1018      -361     

Flag	Coverage Δ
unittests	72.69% <ø> (+24.88%)	⬆️

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