Add documentation for Search for unlinked local files

[rocket125]

Is your suggestion for improvement related to a problem? Please describe.
Lookup -> Search for unlinked local files is not documented.

Describe the solution you'd like
A new page in the documentation under "Collect" should have "Search for unlinked local files" as a section describing how to use it.

Additional context

[koppor]

This is https://docs.jabref.org/collect/findunlinkedfiles#using-the-wizard-search-for-unlinked-local-files.

--

Maybe, "just" the heading should be changed?

We are aiming to have the documentation centered around use-cases not the excact strings in JabRef. But still make the strings discoverable.

I wonder why you did not click on the string ending with "PDFs."... Maybe it should be "Add entries based on local fiels"?

---

Note that the search points to this page