style(sidebar): expand sponsorship banner on specific pages (backport #957)

[mergify]

Basic Info

Info	Please fill out this column
Ticket(s) this addresses	N/A
Does this PR contain AI-generated software?	No

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Description of contribution in a few bullet points

• Added custom metadata property that allows expanding the sponsorship banner on specific pages. The banner will now be displayed by default on pages without a table of contents or when the property is set:

  ---
  expand_sponsorship_banner: true
  ---
  

• Displayed a full-size banner on the main index pages:

  • Home
  • Robots Using
  • About & Contact

  Note: The banner hides their tables of contents.
  On other main pages, the banner is expanded by default.

• Simplified logic for display conditions.

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This is an automatic backport of pull request #957 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of e127b5d has failed:

On branch mergify/bp/lyrical/pr-957
Your branch is up to date with 'origin/lyrical'.

You are currently cherry-picking commit e127b5d.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   overrides/assets/stylesheets/sponsorship_banner.css
	modified:   overrides/main.html

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   docs/about_and_contact/index.md
	deleted by us:   docs/index.md
	deleted by us:   docs/robots_using/index.md

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[mergify]

@mergify[bot], your PR has failed to build. Please check CI outputs and resolve issues.

[SteveMacenski]

@AJedancov this is a good question - we don't need to update the main pages, but does the css and overrides get shared for updating the other pages too?