Document share link protection feature of brute force protection app

[karakayasemi]

The application config parameters still the same and they now apply to both login and share link protection. We may want to mention in docs that public link passwords are also under protection now. Optionally, the screenshot of the app given on the doc page can be updated.

What Needs to be Documented?

https://github.com/owncloud/brute_force_protection/pull/90

Where Does This Need To Be Documented?

https://doc.owncloud.com/server/latest/admin_manual/configuration/server/security/brute_force_protection.html

What Type Of Content Change Is This? (Optional)

• [x] New Content Addition

Which Manual Does This Relate To? (Optional)

• [x] Admin Manual