Implement policy for reviewing/merging mdn content PR's

[escattone]

Implement the policy for handling mdn/mdn-content repo's PR's decided in #1097.

[peterbe]

Here's a start:

1. If the PR has merge conflicts, it's on the author to resolve and merge in main to her branch and push that updated branch.
2. If CI fails, it won't be merged. The editor and author will share the responsibility to find out why CI failed.
3. Don't use git rebase on a branch that has an associated PR created.
4. The editor has the right to close PRs that are deemed to span too many unrelated topics if the PR isn't phrased as a general solution that explains the breadth of the change.
5. If it's a non-trivial edit, there should exist an issue and that issue's ID (e.g. #123) should be mentioned in the PR description.
6. Don't reopen a PR that an editor has closed.

[escattone]

@peterbe I'll add your items to the content contribution guide (README.md), and then I'm going to close this, since we've got the CODEOWNERS file in place as well a contribution guide. We can work from there in terms of tweaking the contribution guide going forward.