Fix scrollbar

[yucheng11122017]

What is the purpose of this pull request?

• Documentation update
• Bug fix
• Feature addition or enhancement
• Code maintenance
• DevOps
• Improve developer experience
• Others, please explain:

Overview of changes:
#2217
Changed the property value of overflow-y of the .nav-component to scroll to allocate space to the scrollbar as discussed here so that its appearance and disappearance doesn't cause the icon in the nav bar to shift.

Since this change is to .nav-component, both the sitenav and pagenav is affected.

Since the change is to the main.css file of the default template, all the other main.css files also have to be updated. Currently there isn't a way to automatically update the file after a MarkBind upgrade (#1961), so if this change is accepted, there has to be release note for current users to update their main.css file.

Anything you'd like to highlight/discuss:
overflow-y: overlay is a legacy value alias for overflow-y: auto according to the mdn docs here, which unfortunately still causes the moving of the content from what I can see.

The overlay value of the overflow CSS property overall is being deprecated and standardised as the scrollbar-gutter property, but I see that its not really supported on Safari browser. If anyone wants me to look into using this property instead, or any other alternatives, I'll be happy to try it out.

Previous behaviour:

Screen.Recording.2024-04-03.at.12.56.40.AM.mov

Current behaviour:

Screen.Recording.2024-04-03.at.12.57.56.AM.mov

Testing instructions:
Serve the docs using markbind serve -d, collapse all sections of the siteNav, open the sections one by one until the scrollbar appears. Ensure that when the scrollbar appears, the icons doesnt shift.

Proposed commit message: (wrap lines at 72 characters)
Allocate space for scrollbar in nav components

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Checklist: ☑️

• Updated the documentation for feature additions and enhancements
• Added tests for bug fixes or features
• Linked all related issues
• No unrelated changes

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Reviewer checklist:

Indicate the SEMVER impact of the PR:

• Major (when you make incompatible API changes)
• Minor (when you add functionality in a backward compatible manner)
• Patch (when you make backward compatible bug fixes)

At the end of the review, please label the PR with the appropriate label: r.Major, r.Minor, r.Patch.

Breaking change release note preparation (if applicable):

• To be included in the release note for any feature that is made obsolete/breaking

Give a brief explanation note about:

• what was the old feature that was made obsolete
• any replacement feature (if any), and
• how the author should modify his website to migrate from the old feature to the replacement feature (if possible).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 51.14%. Comparing base (69ec838) to head (625134d).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2500   +/-   ##
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  Coverage   51.14%   51.14%           
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  Files         124      124           
  Lines        5359     5359           
  Branches     1152     1152           
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  Hits         2741     2741           
  Misses       2328     2328           
  Partials      290      290           

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[EltonGohJH]

LGTM