Add documentation in quay plugin to explain new credential settings in proxy backend (backstage 1.28)

[karthikjeeyar]

What do you want to improve?

Add documentation around new [Optional] credential settings in backstage 1.28 to explain different settings available to the users.

See Backstage 1.28 release notes about the BREAKING: Proxy backend plugin protected by default

• https://github.com/backstage/backstage/releases/tag/v1.28.0

What does the current documentation state?

Current documentation does not have this section.

What would you like to be added/updated in the documentation?

Add a note about this new credentials settings in quay plugin.