ci(v2): remove duplicated lint jobs/steps

[galargh]

• Because this PR includes a bug fix, relevant tests have been included.
• Because this PR includes a new feature, the change was previously discussed on an Issue or with someone from the team.
• I didn't do anything of this.

---

This PR is a subset of #5301.

In this PR, I minimise the number of places where we perform linting to limit duplication. In particular:

1. I changed the trigger of the Lint workflow to pull_request_target and added a second job that comments on a PR when needed.
2. I removed Comment on lint failure workflow because it is duplicated by the Lint workflow after the changes.
3. I removed the linting step from the Check Docs Site workflow because it is duplicated in the Lint workflow.

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[alcuadrado]

Out of curiosity: what's the deal behind this double checkout?

Is the first one for pushes to main and the other one checking out the merge commit of a pr?

Maybe leaving a comment would be good for future readers.

[galargh]

I missed an if here. The first checkout should only be executed if we're not running in the pull request context. Thanks for catching this 🙇

[galargh]

FYI, after resolving the conflicts I had to restore 2 checkouts when we're running on pull_request_target. This is so that we can run lint on the code from the pull request but get the code of the setup-env action from the base branch.

I added the following comment above the first checkout so that this bit of context is not lost:

      # NOTE: When running on a pull_request_target, we're checking out the repo
      # twice. The first checkout is for the base branch of the PR. The second,
      # for the merge commit. We do this to be able to use the `setup-env`
      # action that's on the base branch.