[ext.pages] Add files parameter to Page object

[krittick]

Summary

This adds a new parameter files to the Page object, allowing users to include a list of files to be shown for that page alongside its content/embed(s).

Closes #692

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• If type: ignore comments were used, a comment is also left explaining why
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, typehinting, examples, ...)