What is this feature?

Adds additional info to custom branding docs about the login_logo config option (such as size and file type recommendations and how to reference locally stored images as the logo).

Why do we need this feature?

There have been several support escalations regarding these, therefore they would benefit from additional clarification.

Who is this feature for?

All users of the custom branding docs

Which issue(s) does this PR fix?:

Fixes https://github.com/grafana/support-escalations/issues/17573, https://github.com/grafana/support-escalations/issues/13589

Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.