[Documentation] Add missing default extension points

[scrhartley]

Description

On the page Building htmx Extensions it includes an incomplete list of the extension points available to override. These are init, which I can see is used by official htmx extensions, and getSelectors which has a test covering its usage.

Testing

Coped from source code, which is already covered by tests.
I cross references this with how they're invoked to make sure I wasn't missing any parameters.

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  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

[Telroshan]

Good catch! This was indeed an omission, thanks