docs(material): add autocomplete loading example

[hadt0786]

docs(material): add autocomplete loading example

[hadt0786]

Summary

• add an asynchronous autocomplete example with a visible loading state
• show a disabled "No results found" option when the search completes without matches
• use switchMap to cancel stale searches and finalize to clear loading state
• document the pattern without changing the MatAutocomplete API

Fixes #18478

Testing

• documentation/example-only change; CI will validate the example and docs build