Add help document describing intended wiki linking behavior

Related to #16568
Closes #18019

- This documents the _intended_ behaviour of linking to wiki pages/files.

## TODO

- [x] Respond to @rymai's comments
    - [x] Use `succeed`
    - [x] Shouldn't this be <wiki_root>/miscellaneous.md here?
    - [x] what happens for file links that starts with a `/`?

See merge request !4372