Support relative links from a wiki page to files in the repository
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- Related to https://gitlab.com/gitlab-org/gitlab-foss/-/issues/3275
# Problem
- There's no way to create a relative link from a wiki page to a file in the same repository
- Previously, links of the form `[README.md](../blob/master/README.md)` from the wiki worked as expected
- This was a side effect of a lax implementation and was incidentally removed in https://gitlab.com/gitlab-org/gitlab-foss/-/issues/18019 / https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/4432
# Proposal
- Allow linking to repository files from a wiki page using syntax like `[README.md](../blob/master/README.md)`
issue