docs: for 2.x, fix site title and unstyled error pages

[dwilding]

After merging #1942, I noticed a couple of issues with the 2.x docs:

• The site title should be "Ops 2.x documentation" not "Ops 2.23 documentation"
• Error pages are still unstyled (example). Unfortunately I can't test fixes without building the live RTD docs. But I think this PR should fix it - we'll find out after it merges

[dwilding]

Thanks for the reviews on this and the earlier PRs!

This didn't seem to fix the error pages though. But I won't spend more time on this now. For the 3.x docs, I'm going to update to the latest starter pack, so I'll try to fix it for those docs. Then I'll backport the fix here if I can.