Two FAQ proposals - disable plugins and no file or directory

[Two FAQ proposals](https://beets.readthedocs.io/en/latest/faq.html):

# How do I...

Temporarily disable plugins? (A: Not sure about this one. Setting plugins to `''` (2 single quotes) seems to work.)

# Why does beets…

Complain about "no such file or directory" (A: Run beet update with a query if you have removed files without using beets)

If these questions make sense for the FAQ I will write them up.

Start with the existing FAQ at the linked Read the Docs page and review how its entries are structured. Verify the two proposed questions and answers against current beets behavior, then add them if appropriate; done means both entries are accurate and integrated into the FAQ.

Written by the indexing model from the issue text.