Add docs for changing cluster plan between Standard and Basic

[katmayb]

Fixes DOC-10627

This PR adds a page to describe changing cluster plans between Standard and Basic clusters.

Also:

• Changed the language from "migrate" to "change" on the Standard to Advanced page.
• Added a short section on the cluster management pages for Basic and Standard to direct customers to the new page.
• Updated some errors here and there.

Preview

New page: https://deploy-preview-19462--cockroachdb-docs.netlify.app/docs/cockroachcloud/change-plan-between-basic-and-standard.html