docs(extensions): quick-start.md references a non-existent 'query insights' example extension

### Problem

\`docs/developer_docs/extensions/quick-start.md\` (around line 516) tells readers to "examine the query insights extension in the Superset codebase" as a worked example. That extension doesn't exist anywhere in the repo — searched the whole tree, no match.

Right now there's no working end-to-end reference extension in-repo at all (the community registry entries in \`registry.md\` are all third-party and mostly SQL-Lab panels), so this dead reference leaves new extension authors with nothing concrete to model against.

### Suggested fix

Either point to a real example (if one exists elsewhere / is planned), or remove the reference and note that none exists yet.

Found while researching the extensions system for apache/superset#24271 (SIP-96).

Open docs/developer_docs/extensions/quick-start.md around line 516 and verify the referenced query insights extension against the repository. Review registry.md and the extensions documentation to determine whether a real in-repo example can be named; done means the quick-start no longer points to a nonexistent extension and accurately states the available examples.

Written by the indexing model from the issue text.