PagesDomain: allow update of key/certificate

What does this MR do?

• Adds the feature to update the TLS key/certificate of a given PagesDomain, without having to delete and re-create the domain instance from scratch.
• Slight improvements to domain breadcrumbs in Pages section. (Add a Pages breadcrumb, and use the domain's hostname as the breadcrumb title.)

Are there points in the code the reviewer needs to double check?

The UX is implemented slightly differently from the proposal in the original issue. Specifically a new edit action has been added, separate to the pre-existing show action. I felt this was both easier to implement, and maybe a little more intuitive from a user's perspective.

I'd be interested to hear the thoughts of others, and happy to consider changes here.

Why was this MR needed?

Closes #29720 (closed).

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?