Components: Update Autocomplete usage example

[chad1008]

Fixes #16624, #33852

What?

Updates the Autocomplete component README file with more a more useful description and example.

Why?

The current example gives a basic picture of the components functionality, but doesn't work on its own, in large part because Autocomplete is tightly coupled with other packages/components like the block editor and RichText

How?

The updated README clarifies that Autocomplete isn't intended as a standalone component and provides an example of how to integrate new completers in the WordPress Block Editor.

Testing Instructions

Using the examples provided, try creating a small plugin, and confirm that it functions as expected without any errors.

[github-actions]

Flaky tests detected in 9ca7134.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/4758305626
📝 Reported issues:

• [Flaky Test] should switch to Template tab when a block was selected and we select the Template #49078 in specs/site-editor/settings-sidebar.test.js

[mirka]

Makes sense and works as expected 👍