Revamp CI/CD cache reference docs

What does this MR do?

Related to #333941 (closed)

Updates the CI/CD cache reference material to use the new style: https://docs.gitlab.com/ee/development/cicd/cicd_reference_documentation_guide.html

Moves the Use multiple caches section and the Fallback cache key section to /doc/ci/caching/index.md. As a result, I moved the two Share caches... example sections down to the #common-use-cases section, which seemed to work better after I copied the other sections over.

I also adds some crosslinks that make it easier to find additional examples and related information.

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