[CI] Consider refactoring comments into a separate workflow

[novusnota]

There should be a comment at the start of each PR that will give a lot of useful info regarding that PR.

It should:

1. Greet new contributors
2. Give links for all edited pages in the PR to their corresponding staging URLs
3. List all possible commands, including those that are suggested in the AI workflow (/review, /fmt)
4. Give links to contributing materials, style guides in particular

The idea behind this workflow is to help reviewers by providing direct links to the deployed pages, help newcomers by explaining the contributing workflows (incl. CI), and help maintainers keep the quality bar high.

Prior notes

The workflow_call can allow to separate some the logic of checks from the "how to fix me" comments, effectively re-using the same commenting logic for any such needs.

Some CI checks, such as formatting, path name validation, image component checks, or the redirect checker against the upstream links could all post comments on fixing them. Since we now have .github/scripts/common.mjs, let's just reuse that code outside of forks.

As an aside, we can check how well the issues are opened (#1488) or convert PRs to drafts once they receive any review comments (debatable).

Done:

• Passing checks, which should hide all prior "how to fix me" comments from the bot, see [Format] If the formatting check was successful, it should hide all prior comments from the bot and mark those as resolved #1284

Handled elsewhere:

• New comments from maintainers in PRs. For example, the /fmt comment: feat(ci): revamp JSX-in-MDX formatting and add /fmt command for maintainers #1263