add mariadb image.registry fallback to docker.io and update docs for mariadb.image parameters

[jessebot]

Pull Request

Description of the change

This just adds some docs for mariadb.image and adds a fallback for the image.registry parameter.

Benefits

This just makes postgres and mariadb a bit more consistent and adds docs

Possible drawbacks

we can't reasonably maintain the image tag in the docs, because it would change everytime we update the subcharts, so I've left it blank.

Applicable issues

This fixes the issue brought up in https://github.com/nextcloud/helm/pull/471#issuecomment-1848316834

Additional information

Checklist

• [x] DCO has been signed off on the commit.
• [x] Chart version bumped in Chart.yaml according to semver.
• [x] (optional) Variables are documented in the README.md