fix: set startKey for _design_docs next page

[ricellis]

PR summary

Fix _design_docs pagination

Fixes: part of s1082

Note: An existing issue is required before opening a PR.

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) - discovered in WIP gauge tests
• 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?

Design docs paging sets a key instead of a start_key so can only get 1 page + 1 doc

What is the new behavior?

Set the start_key to paginate correctly.

Does this PR introduce a breaking change?

• Yes
• No

Other information