Fix Docs Panel

[josephjclark]

• Tweak the margins and scrollbars inside the panel so that it looks a little better in small windows (it's always a small window when it opens by default)
• Restore overflow: hidden from #2154, which seemed to get dropped somewhere

Closes #2195

QA notes

This needs really thorough testing!

• Check the docs panel retains a reasonable size when the left, right and center columns are expanded
• Check the docs panel retains a reasonable size when expanded and collapsed, in horizontal and vertical layout
• Check the docs panel retains a reasonable size while the browser window is resized and zoomed

Note that if the docs panel is very narrow, some tabs and controls will be hidden from view. This is expected and there is nothing we can do about it.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.70%. Comparing base (f8e9c7f) to head (9c6f51e).

:exclamation: Current head 9c6f51e differs from pull request most recent head a1f1ed9

Please upload reports for the commit a1f1ed9 to get more accurate results.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2198 +/- ## ======================================= Coverage 89.70% 89.70% ======================================= Files 275 275 Lines 9039 9039 ======================================= Hits 8108 8108 Misses 931 931 ```

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