[Next Docs] Update Git Workflow (#50579)
Update our git workflow in preparation for open-sourcing the content of
the docs ([linear
task](https://linear.app/vercel/issue/DX-1579/set-up-github-workflow)).
**Templates:**
- [x] Update docs issue template to encourage contributions
- [x] Update PR template to include link to new contribution guide
**Code Owners / Reviewers:**
- https://github.com/vercel/next.js/pull/50841
**Labels:**
- [x] Add DevEx team to labeler.json so PRs get the "created by: DevEx
team"
**Other:**
- [x] Remove docs manifest from CI checks as we no longer have one (keep
the manifest for errors as they live under `/pages`)
- [x] Add `unifiedjs.vscode-mdx` to the vscode extension list
