Add Page class to ext.pages to allow for greater flexibility with page contents

[krittick]

Summary

This adds a new class, Page to ext.pages, which allows users to specify both the content and embeds attributes. This allows the paginator to display pages with both content and embeds, where previously it only allowed one or the other on each page.

Existing usage of passing lists of strings or embeds still works as it did before.

This is being added as part of 2.0 to allow for a much easier guide to be written for end users rather than trying to have them understand the nuances of passing a list of embeds, list of list of embeds, or list of strings. Now they can just define their Page objects and pass a list of those.

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, ...)