add readthedocs content to the guide

[miguelgrinberg]

This change adds content only available in the readthedocs documentation to the Python client guide hosted on elastic.co.

• new section on using asyncio with the Python Elasticsearch client
• additional examples hosted on the elasticsearch-labs repo as notebooks
• minor additions and improvements to the getting started page

[github-actions]

A documentation preview will be available soon.

• 🔨 Buildkite builds
• 📚 HTML diff
• 📙 Preview page

Request a new doc build by commenting

• Rebuild this PR: run docs-build
• Rebuild this PR and all Elastic docs: run docs-build rebuild

_{run docs-build is much faster than run docs-build rebuild. A rebuild should only be needed in rare situations.}

_{If your PR continues to fail for an unknown reason, the doc build pipeline may be broken. Elastic employees can check the pipeline status here.}

[pquentin]

Excellent work,thank you! Can we easily add a sample code snippet to the index page?

[pquentin]

Thanks! LGTM.