Reorder and rename sections in variables doc

What does this MR do?

As part of the related issue, begins to get sections into their final locations within the CI/CD variables doc. This MR:

• Renames sections as per current standards.
• Shuffles the sections around a bit so that the groupings are better (as well as nesting).
• Removes a handful of superfluous headers.
• Removes outdated/unneeded screenshots.

See screenshots below to get an overview of the reorganization. It does not change the content in most areas, though there are some tweaks to make the sections work in the new places, or to make sure content isn't lost when we removed the extra headers.

• Current: https://docs.gitlab.com/ee/ci/variables/
• Review app: https://main-ee-107808.docs.gitlab-review.app/ee/ci/variables/

The unfortunate side effect of trying to improve one of the most crosslinked pages in the docs is that renaming headers results in a lot of broken anchor links. This MR also updates all those links, and sometimes removes the anchors when the crosslink is about variables in general and doesn't need to link to a specific section.

Finally, we also crosslink to some of these sections from the UI, so we will need a frontend maintainer to do the final merge.

Screenshots

Before	After

Related issues

Related to #382363 (closed)

Author's checklist

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