BUG - center search dialog on wide screens

[gabalafou]

I noticed while reading the docs that on very wide screens, the search dialog was off center.

This PR gets rid of all unnecessary CSS that was left over from when the search dialog was implemented without the built-in benefits of the HTML modal dialog element.

How to test

1. Go to the preview docs build for this PR.
2. Make your screen really wide > 2000px.
3. Click on the search button in the header nav.
4. The search dialog should be centered now.
5. For good measure, type something in the search bar and hit enter.

[gabalafou]

I tested this PR locally on my machine on Firefox, Safari, Chrome. I also tested on the iOS simulator.

[drammock]

Thanks, this had been bugging me!

[github-actions]

Coverage report

This PR does not seem to contain any modification to coverable code.

[kaycebasques]

(Just found this PR)

The search-as-you-type UX I contributed in #2093 conflicts with this

[drammock]

I think I've resolved conflicts correctly, but RTD build is still stalled by #2083 😢 so I can't check the PR build to make sure things still look OK

@kaycebasques can you check the diff here to make sure I didn't bungle the merge / kept the important parts from #2093?

[drammock]

well the centering looks good, but I seem to have lost the opacity of the search results popup container: https://pydata-sphinx-theme--2085.org.readthedocs.build/en/2085/