Remove underscores from breadcrumbs

[katjuell]

In #2066 we saw several things happening: a parent directory was swapped into the URL for links (when a trailing slash was present), adding an additional term, and breadcrumbs had underscores. The missing index page issue was resolved in #2105

Underscores shouldn't be present in breadcrumbs or URLs, given the redirects we have in place. If, however, there are any, this PR will remove them. It also removes lingering references to underscored files in use_cases.yml (part of the larger effort of #1932 to standardize file names).

[netlify]

✔️ Deploy Preview for sourcegraph-handbook ready!

🔨 Explore the source changes: bb96d22

🔍 Inspect the deploy log: https://app.netlify.com/sites/sourcegraph-handbook/deploys/62055222caaa9d0008befb59

😎 Browse the preview: https://deploy-preview-2296--sourcegraph-handbook.netlify.app

[github-actions]

Hey there! It looks like this pull request has been sitting for a long time. Keep in mind that when you make a pull request, the change doesn't actually go live until you merge the pull request. This needs to be done by you (the pull request author) as soon as you feel like you got the appropriate amount of review/approvals that make you confident with the change. For example: adding yourself to the team page doesn't require anyone's review and you can merge directly, while when proposing a change to your team's processes you'll probably want to get your team's review (and need to request review from your team explicitly, so they get notified).

Please see whether you want to merge this pull request, close it and discard the change, or remind the people you would like review from.

You can find more help on our Editing the handbook handbook page.

Thank you! \(^-^)/

[github-actions]

Since there were no updates in the past 7 days, this pull request is being automatically closed.
If this is a mistake, you can simply click the Reopen button to reopen the pull request.

Thank you! \(^-^)/