Add cross-project wiki references

What does this MR do and why?

This adds cross-project and cross-namespace references for wiki pages:

Type	Local	Cross-project	Cross-namespace
Wiki page (if the page slug is the same as the title)	[[Home]]	[[namespace/project:Home]]	[[namespace:Home]]
Wiki page (if the page slug is different from the title)	[[How to use GitLab|how-to-use-gitlab]]	[[How to use GitLab|namespace/project:how-to-use-gitlab]]	[[How to use GitLab|namespace:how-to-use-gitlab]]

While this is by itself of practical use to users, it will be used as a stepping stone for Wiki page mentions show as plain text instead o... (#550296 - closed).

References

• Add explicit cross-project references to projec... (#550305 - closed)
• Wiki page mentions show as plain text instead o... (#550296 - closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Go to a Markdown input (editing descriptions, adding a comment).
2. Try out the documented cross-namespace references.

MR acceptance checklist

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