Paginate the GET /api/resources endpoint

[textbook]

This is a:

• ✨ New feature - new behaviour has been implemented
• 🐛 Bug fix - existing behaviour has been made to behave
• ♻️ Refactor - the behaviour has not changed, just the implementation
• ✅ Test backfill - tests for existing behaviour were added but the behaviour itself hasn't changed
• ⚙️ Chore - maintenance task, behaviour and implementation haven't changed

Description

• Purpose - paginate the resource list in the API to keep the system performant as the number of resources increases

• How to check -
  • Make sure you have at least 21 published resources in your dev environment
  • Visit the home page
  • Pagination controls should appear below the resource list
  • The controls should work correctly
    • Moving to next and previous page
    • Deactivating next/previous as appropriate
    • Moving to specific page
    • Changing page size
  • Review the API docs, /api/resources endpoint descriptions should match the observed behaviour

Links

• Drafts aren't yet paginated, so only 20 get shown, but Allow admins to reject resource suggestions #13 will allow that list to be kept clear

Author checklist

• I have written a title that reflects the relevant ticket
• I have written a description that says what the PR does and how to validate it
• I have linked to the project board ticket (and any related PRs/issues) in the Links section
• I have added a link to this PR to the ticket
• I have made the PR to main from a branch named <category>/<name>, e.g. feature/edit-spaceships or bugfix/restore-oxygen
• I have manually tested that the app still works correctly
• I have requested reviewers here and in my team chat channel

• I have spoken with my PM or TL about any parts of this task that may have become out-of-scope, or any additional improvements that I now realise may benefit my project
• I have added tests, or new tests were not required
• I have updated any documentation (e.g. diagrams, schemas), or documentation updates were not required