Expand yaml reference for yaml options and value

What does this MR do?

This MR expands the YAML keyword reference with sections specifically for variables: value and variables: options, which previous didn't have their own section (so we couldn't link to them). I have also added some additional details that I noticed while testing this.

I changed the example in the ci/pipelines/index.md page as well, as the old example would be better done as a downdown list with variables:options instead. The updated version shows two use cases that don't need options.

Related to this (and the related issue), now that we have them in the reference, we can link directly to them from the CI schema, so I've updated that as well.

Related issues

Resolves #385015 (closed)

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