Improve docs structure (#467)
Improve docs structure (#467)

- Creates a tree structure for documentation, it allows to organize it better
  from a developer point of view and also the rendered documentation should be
  easier to navigate.
- Moves partially the main readme to be included in the online docs, the main
  readme will be updated to have a link to avoid duplicated content)
- Moves the examples folder to the docs, so they can be accessed through the
  online documentation. Creates a new pair of "macros" to create links to
  specific versions, scm_web & scm_raw_web.

Co-authored-by: Chris Kleinknecht <cb9c5e2d56e129ddbf7d7f021e9ecdcc26174648@google.com>
open-telemetry/opentelemetry-python-contrib,open-telemetry/opentelemetry-python-contrib,open-telemetry/opentelemetry-python-contrib