Cleanup example code in docs

Changes

Adds:

• Examples pages in docs build. Note: the pages are only linked in index.rst and not the README.md file, since there doesn't seem to be a good way for the md parser to redirect the links to the sphinx toctree when ingesting a file via "include" statements using a non-default parser. TL;DR: tried linking to examples in README, didn't parse correctly.

Changes:

• Fixes docs Quickstart section, there were some stale pieces of code in the example. The new text is copied and pasted from the test_examples module, which verifies that the code runs correctly.
• Fixes a type default on IterFrame class. Neither CI suite nor @james.kennington able to reproduce error reported by @yun-jing.huang, but @patrick.godwin verified that mypy can provide a warning. Included on this MR since not affecting production spec / tests, the default value being identical to parent class instead of unspecified is effectively a docs change.