docs: Add Collapse and Placeholders examples to Cheatsheet Example

[coliff]

Description

Introduced new sections for Collapse and Placeholders components in the cheatsheet example page, including navigation links and example usage for each component.

Ideally we'd update the RTL page too but I don't have translations for the components. Shall we use auto-translate for it?

Motivation & Context

Cheatsheet page is really useful to see all the components on a single page and Collapse and Placeholders were missing.

Type of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would change existing functionality)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

Live previews

• https://deploy-preview-42043--twbs-bootstrap.netlify.app/

[Copilot]

Pull request overview

This PR adds two missing Bootstrap component sections (Collapse and Placeholders) to the cheatsheet example page, making it more comprehensive for developers who use it as a quick reference.

Changes:

• Added navigation links for Collapse and Placeholders components in the sidebar menu
• Added Collapse component section with standard toggle example
• Added Placeholders component section with card skeleton loader example

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[coliff]

I think some of the failing tests are unrelated to my changes.

command used: java -jar "/home/runner/work/bootstrap/bootstrap/node_modules/vnu-jar/build/dist/vnu.jar" --asciiquotes --skip-non-html --Werror --filterpattern "Attribute “autocomplete” is only allowed when the input type is.*|
Attribute “autocomplete” not allowed on element “button” at this point.|
An “aria-disabled” attribute whose value is “true” should not be specified on an “a” element that has an “href” attribute.|Attribute “is:raw” is not serializable as XML 1.0.|
Attribute “is:raw” not allowed on element “code” at this point.|
Trailing slash on void elements has no effect and interacts badly with unquoted attribute values.|
Attribute “switch” not allowed on element “input” at this point." _site/ js/tests/
"file:/home/runner/work/bootstrap/bootstrap/_site/docs/5.3/examples/cheatsheet/index.html":1141.13-1141.52: info warning: Empty heading.

[julien-deramond]

"file:/home/runner/work/bootstrap/bootstrap/_site/docs/5.3/examples/cheatsheet/index.html":1141.13-1141.52: info warning: Empty heading.

Can't run it locally right now, but it might come from the following empty heading:

<h5 class="card-title placeholder-glow">
  <span class="placeholder col-6"></span>
</h5>

[coliff]

thanks for that pointer! I noticed the Placeholder example actually uses the following code, so I updated it and squashed the commits:

<div class="h5 card-title placeholder-glow">
  <span class="placeholder col-6"></span>
</div>