web ui: New setting for browser-based login

This is useful in cases where OIDC login is active
which would then redirect the user to a central
login page.

See https://github.com/Kozea/Radicale/issues/2143

DOCUMENTATION.md

@@ -1699,6 +1699,15 @@ Available backends are:
 
 Default: `internal`
 
+##### prefer_browser_login _(>= 3.7.4)_
+
+Try to use browser based login rather than the built in login in the web UI. This 
+can be helpful if the login system you are using uses redirects, such as in
+an OIDC setup. For this to work properly, you will also have to make sure CORS
+headers are configured correctly.s
+
+Default: `False`
+
 #### [logging]
 
 ##### level