Complete CTRT rewrite of approvals page

What does this MR do?

Revises the index page for the new merge request approvals section. I split this page apart in !60447 (merged) which landed earlier today.

This merge request...

• Revises the introduction substantially
• Brings the page up to GitLab tone and style
• Strips some old images that could be better communicated with text, with less potential technical debt when the UI (inevitably) changes
• Adds a related-links section to more clearly link to the API and admin-area pages

This merge request does NOT crosslink much to the other two pages (settings.md and rules.md in the same directory), because I'm about to rewrite those and many subheads need renaming. I'd like to skip the crosslinking until after I tidy those two pages up; otherwise I'll be redoing work.

What I need from @danielgruesso

• Look for factual errors. I can't guarantee I didn't muck up a detail or two.
• Look at the title of the page and confirm the tier availability is correct.
• Look at each of the subheadings. If they have a different tier availability than the title of the page, is the subheading badged correctly?

Related

Related to #324948 (closed) which tracks the entire need

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #324948 (closed)