Fix docs custom 404 page and reword messaging

[yzdocs]

Fixes #4908 and #8076.

The custom 404 image is a stretch goal so I haven't attempted it with this PR.

Please preview the new 404 page here: https://deploy-preview-8415--cockroachdb-docs.netlify.app/docs/stable/404.html

You will not be able to see the page above by going to a broken link via preview site, because the way Netlify redirects, it will send you from a preview broken link to the 404 page in production. But you can use the Netlify preview site to verify that broken links are being redirected to the 404 page in production, which is an improvement over current behavior (see #4908).

I think this is one PR that we should make live immediately instead of waiting for all style updates. This is because the current 404 functionality is broken (see #4908), so we are essentially fixing a bug that affects users today.

[cockroach-teamcity]

This change is 

[jseldess]

Thanks, @yingzhuchin!

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @anagio, @jseldess, @lnhsingh, and @luciaaaaaaa)

[lnhsingh]

and I agree that it makes sense to push this one live

Reviewable status: complete! 2 of 0 LGTMs obtained (waiting on @anagio, @lnhsingh, and @luciaaaaaaa)

[luciaaaaaaa]

Wondering if we want to add the 'Return to home' button, as a way for the user to have a clear action to take from here. Otherwise, lgtm!

[yzdocs]

Wondering if we want to add the 'Return to home' button, as a way for the user to have a clear action to take from here. Otherwise, lgtm!

I think since the search bar, sidebar, and top navigation bar are all still accessible on the 404 page, the user should be good on what to do next. My two cents!

[yzdocs]

I'll merge this for now since I'm shifting focus, but we can add any additional asks to #8082 and pick up from there!