Revise the subheadings about state file UI

What does this MR do?

This merge request is intended to support the release post item gitlab-com/www-gitlab-com!71446 (merged) by revamping the section about managing terraform state through the UI. Revamping the full page is out of scope of what can be done before %13.8 lands, but I can bring this section of the page closer to the concepts-tasks-resources format we're supposed to adhere to.

What this MR does:

• Adds introduced-in information to the UI section. (If we don't, gitlab-com/www-gitlab-com!71446 (merged) will be flagged as a problem prior to the %13.8 release)
• Starts the section by describing the concept of managing terraform state through the UI, and describes the UI.
• Where applicable (the actions menu) crosslinks to more information.
• Clarifies what user levels can view information and take action.
• Combines the subheadings about removing a state file. Doing this through the UI and through the REST API are essentially one-liners and don't need subheadings. The GraphQL version is long enough to keep in a subheading.
• Some cleanup of wording issues identified by Vale.

Related issues

Related to gitlab-com/www-gitlab-com!71446 (merged) which is the release post for this item.

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