Update SDK-PR-guide.md to include branch lockdown label

[marcpopMSFT]

I included a comment in the PR guide about the lockdown label
Additionally, I added a workflow to automatically add that label.

• The branch is generally open between the first tuesday of the month when branding goes in until the third tuesday of the month (basically a week after the internal to public merge).
• Outside of that window, let's automatically add the label to any 8/9 base branch PR
• The workflow uses the permissions of the target branch as well as the source for the target branch so won't run until it's merged
• I configured it to run on all PRs but will only check into main.
• When it runs, it'll search all PRs and label them all so we don't need to configure it to run elsewhere.
• I check if it's a PR against the 8 or 9 branches filtering out 9.0.3xx
• I check if it already has the label

[marcpopMSFT]

/backport to release/9.0.2xx

[github-actions]

Started backporting to release/9.0.2xx: https://github.com/dotnet/sdk/actions/runs/13079866389