Add CustardUI script as bundled custom plugin

[gerteck]

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:

Adds CustardUI as a easy to toggle plugin by adding direct support

Closes #2901

Anything you'd like to highlight/discuss:

Testing instructions:

• markbind init a new site, or use any existing site
• Add 'custardui' to the plugins list, (add baseUrl as plugin context for other features)
• markbind serve: try going to url, www.url.com/path#cv-share , (e.g. https://nus-cs2103-ay2526-s2.github.io/website#cv-share), if custardui plugin is enabled, a highlighting toolbar should appear.

Proposed commit message: (wrap lines at 72 characters)

Add CustardUI script as bundled custom plugin

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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 71.32%. Comparing base (6bbfc2e) to head (f0218e7).

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #2906    +/-   ##
========================================
  Coverage   71.32%   71.32%            
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  Files         133      133            
  Lines        7234     7234            
  Branches     1612     1714   +102     
========================================
  Hits         5160     5160            
+ Misses       2068     1973    -95     
- Partials        6      101    +95     

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[Incogdino]

LGTM! Left a comment about the docs, let me know what you think!

[Incogdino]

LGTM!