Defer hx-ext usage to Usage section. Fixes #3467

[davewsmith]

This removes mention of hx-ext from the Installation sections of most of the extension doc pages, deferring to the Usage section for correct attribute placement. This resolves the misleading/incorrect examples on the sse and ws pages, and extends the change to other extension pages for consistency.

Corresponding issue: #3467

Testing

N/A

Checklist

• [x] I have read the contribution guidelines
• [x] I have targeted this PR against the correct branch (master for website changes, dev for source changes)
• [x] This is either a bugfix, a documentation update, or a new feature that has been explicitly approved via an issue
• [ ] I ran the test suite locally (npm run test) and verified that it succeeded
• [x] No LLM was involved in this change