[BD-46] docs: added keep the menu nav section of viewed page expanded

[PKulkoRaccoonGang]

Description

• Ensured the section containing the currently viewed page on the Paragon docs site is persistently expanded such that users know which section they are currently in.
• Ensured the current page in the collapsible menu section receives some "active" styling.

Issue: #2776

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Thanks for the pull request, @PKulkoRaccoonGang!

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (12c5573) 92.83% compared to head (20bdd4c) 92.83%.
Report is 14 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2791   +/-   ##
=======================================
  Coverage   92.83%   92.83%           
=======================================
  Files         235      235           
  Lines        4240     4240           
  Branches     1029     1029           
=======================================
  Hits         3936     3936           
  Misses        300      300           
  Partials        4        4           

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[PKulkoRaccoonGang]

These changes are necessary to comply with the general principles for writing classes in the OpenEdx ecosystem. I noticed that the full name of the class is almost always written to simplify searching in the code

[monteri]

The only thing that I noticed was active items duplication. Other staff is LGTM

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[openedx-semantic-release-bot]

🎉 This PR is included in version 21.7.2 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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[openedx-semantic-release-bot]

🎉 This PR is included in version 22.0.0-alpha.18 🎉

The release is available on:

• npm package (@alpha dist-tag)
• GitHub release

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