fix: DesignDocsPageIterator class modifiers

[ricellis]

PR summary

Fix the class modifiers on DesignDocsPageIterator

Fixes: part of s1082

PR Checklist

Please make sure that your PR fulfills the following requirements:

• The commit message follows the
  Angular Commit Message Guidelines.
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• New tests
• Build/CI related changes
• Documentation content changes
• Other (please describe)

What is the current behavior?

DesignDocsPageIterator is public when it is not meant to be (it is internal and is only meant to be used via Pagination).

What is the new behavior?

DesignDocsPageIterator is marked final (and default package visibility).

Does this PR introduce a breaking change?

• Yes
• No - the class should only be used via the Pagination class and the constructor was not public

Other information