Add web components / shadow DOM example to docs

[kgscialdone]

Description

Added an example explaining how to integrate HTMX with web components and shadow DOM.

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (v2.0v2.0 since this example uses alpha functionality)
• 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

[alexpetros]

This is great, thank you!