Increase page max width to show more content when possible (backport #5523)

[mergify]

This aims to increase the page's maximum width so that it can show more content when possible.

From what I could tell, the current max width is 800px. It's a bit funny to have to scroll horizontally for somewhat-long commands when there's plenty of space, e.g., on an ultrawide 3840×1600 monitor:

This PR adds a custom CSS file that sets the max width to 1000px. The page still correctly adapts if the width is less than that, and this allows us to see more content without having to scroll horizontally.

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I found this relevant issue/discussion: readthedocs/sphinx_rtd_theme#295. There are some valid points against increasing the max width. One of them is that a wider page might decrease reading comprehension: readthedocs/sphinx_rtd_theme#295 (comment). I think this is true, but increasing the width from 800px to 1000px might not be that bad and could help a lot.

It could break things that rely on a specific width, but I've taken a quick look and don't see anything obviously broken.

I opened this as a prototype so that we could discuss it. We could make it bigger if we want, e.g., 1200px, but that might be too much. I thought this might've been discussed before (I sort of vaguely remember something about increasing the page width), but I couldn't find any issue here.

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This is an automatic backport of pull request #5523 done by Mergify.

[github-actions]

HTML artifacts: https://github.com/ros2/ros2_documentation/actions/runs/15171696369/artifacts/3171924809.

To view the resulting site:

1. Click on the above link to download the artifacts archive
2. Extract it
3. Open html-artifacts-5654/index.html in your favorite browser