feat: rewrite same-page links on .plain.html (#656)

[tripodsan]

fixes #642

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (a2182c2) to head (d54f032). Report is 2 commits behind head on 5.x.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 5.x #715 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 50 50 Lines 4477 4480 +3 ========================================= + Hits 4477 4480 +3 ```

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[github-actions[bot]]

:tada: This PR is included in version 5.13.0 :tada:

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