Python: add installation instructions, refine status info, and more

[chalin]

• Contributes to Migrate Python docs to this repo #1109
• Moves various sections of opentelemetry-python/docs/index.rst into the Python instrumentation landing page.
• The corresponding Python repo PR to remove the sections/pages is:
  • [docs] Rework API docs opentelemetry-python#2453

Preview: https://deploy-preview-1116--opentelemetry.netlify.app/docs/instrumentation/python/

/cc @open-telemetry/python-approvers

[chalin]

@lzchen @codeboten @cartermp et all: anything else? Is this ready for a final approval?

[cartermp]

I like it!