Add auto-assign-pr action

[dbreseman]

• Auto-assigns PRs to Technical Writers team members when they create the PRs
• Only assigns if no one is already assigned; does nothing otherwise
• Only applies to Technical Writers team members; does nothing if someone else creates the PR

[MarkvanMents]

Haven't tested it but looks good.
Perhaps mention it at the group meeting and then implement it.
We can always remove it if it gets annoying.
Thanks for picking this up.