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

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  source changes)
• 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
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