MNT Add 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
• Clearly and accurately identify the specific action.

NOTE: Do not merge this until someone confirms they have addd the NETLIFY_BUILD_HOOK secret to this repo.

Parent issue

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

[GuySartorelli]

@maxime-rainville We need access to the webhook to add as a secret for this one.

[maxime-rainville]

LGTM