Rename "Online terminal" to "Web terminal" in the documentation

## What does this MR do?

Renames online terminals to web terminals in some recently merged documentation

## Are there points in the code the reviewer needs to double check?

I think I've caught all the links

## Why was this MR needed?

We're calling it 'web terminal' in the release post, and consistency is ✨ 

https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/4007/diffs#note_20283597

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !8190