Adds common examples for the simpleion load/dump APIs.

[cheqianh]

Some customers found that the lack of documentation makes them really hard to onboard Ion. While discussing and documenting a long-term solution, I'll use this PR to add common examples for the simpleion load/dump APIs first.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[rmarrowstone]

This is a good start.

A couple other observations/suggestions:

1. The main confusion I have as a user is load vs loads I would clarify that somewhere.
2. To that end: we have a module pydoc. Consider enhancing that to aim users to the primary entry points.
3. After the addition of the c-extension it looks like the bulk of the pydoc is now on the _python methods.
4. Personally, I find the type mapping tables (on both loads and dumps) to be too much there and just sort of confusing. A possible suggestion would be to pull that out into the module doc in a single table that had both the Python -> Ion and Ion -> Python type mappings.