Fix redirect paths to _index.md files

What does this MR do and why?

I noticed the redirects pointed at _index.md files are incorrect:

For example, https://docs.gitlab.com/user/group/epics/epic_work_items/ redirects to https://docs.gitlab.com/user/group/epics/_index/ instead of https://docs.gitlab.com/user/group/epics/#epics-as-work-items.

Also removed trailing slashes from anchors, but this was a guess.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment.
2. Check out this branch.

Merge request acceptance checklist

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