python api docs: improve async usage description

[jreiberkyle]

Decrease the barrier to entry for users by providing robust async usage description in the docs that includes code snippets they can drop into their sync code, briefly discusses gotchas, and links to external resources to learn more.

Section to update: https://planet-sdk-for-python-v2.readthedocs.io/en/latest/python/sdk-guide/#create-your-first-session