Update docs to link out to VS Code extension

What does this MR do?

Starts the process of linking a few CI docs out to either 1) the new vscode.md page in the gitlab docset, or the relevant parts of https://marketplace.visualstudio.com/items?itemName=GitLab.gitlab-workflow (the official docs for the extension).

@marcel.amirault suggested these three pages, and I've added the parts I'm comfortable with, but I have concerns about one of the pages:

• https://docs.gitlab.com/ee/ci/index.html#references (maybe rename header)
• https://docs.gitlab.com/ee/ci/lint.html (relates to the Validate your GitLab CI configuration.)
• https://docs.gitlab.com/ee/ci/pipelines/ (relates to View the status of your pipeline feature)

• ci/index.md - these changes made sense to me.
• ci/pipelines/index.md I made some guesses, but I think we're OK overall.
• ci/lint.md - This page talks about linting with a completely different tool. To me, this feels like a complete page restructuring, and I think Marcel is better-suited than I am to make the decision about what to do. (If this work needs to spin off, I'm fine with that.)

Related issues

Closes #343552 (closed)

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