docs(examples): improve code teaser in index page

[M-Mueller]

Summary of Changes

Running the very first example in the documentation no longer results in a NameError. Additionally there is a link to the Quickstart for a full example.

Related Issues

N/A

Pull Request Checklist

This is just a reminder about the most common mistakes. Please make sure that you tick all appropriate boxes. But please read our contribution guide at least once; it will save you a few review cycles!

If an item doesn't apply to your pull request, check it anyway to make it apparent that there's nothing to do.

• [x] Applied changes to both WSGI and ASGI code paths and interfaces (where applicable).
• [x] Added tests for changed code.
• [x] Prefixed code comments with GitHub nick and an appropriate prefix.
• [x] Coding style is consistent with the rest of the framework.
• [x] Updated documentation for changed code.
  • [x] Added docstrings for any new classes, functions, or modules.
  • [x] Updated docstrings for any modifications to existing code.
  • [x] Updated both WSGI and ASGI docs (where applicable).
  • [x] Added references to new classes, functions, or modules to the relevant RST file under docs/.
  • [x] Updated all relevant supporting documentation files under docs/.
  • [x] A copyright notice is included at the top of any new modules (using your own name or the name of your organization).
  • [x] Changed/added classes/methods/functions have appropriate versionadded, versionchanged, or deprecated directives.
• [x] Changes (and possible deprecations) have towncrier news fragments under docs/_newsfragments/, with the file name format {issue_number}.{fragment_type}.rst. (Run towncrier --draft to ensure it renders correctly.)

If you have any questions to any of the points above, just submit and ask! This checklist is here to help you, not to deter you from contributing!

PR template inspired by the attrs project.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (89ccb98) to head (9e827aa).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2241 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 63 63 Lines 6889 6889 Branches 1258 1258 ========================================= Hits 6889 6889 ```

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