MNT Fix github action for deploying userdocs

[GuySartorelli]

• Set the branches that activate the action to match the ones used by userhelp according to the userhelp sources json
• More clearly and accurately identify the specific action (main.yml is hardly specific) in case future actions are also added.

Parent issue

• The SS4 userhelp as a bunch of "userguide" instead of the actual title doc.silverstripe.org#246