Documentation for platform developers (python runtime)

Should include overview of the key components of the runtime, the execution flow and design principals in wiki format. Also at least a paragraph of inline documentation at the module and class level.

Particular focus should be given to extending the runtime with storage and model repo plugins. A dedicated wiki page with more "public-facing" documentation for the key interfaces and bits of framework around these plugin mechanisms.

Start by mapping the Python runtime's key components and execution flow, then inspect the module and class boundaries and the storage and model repo plugin interfaces. Done means a wiki overview covers the architecture, flow, design principles, and public plugin mechanisms, with module- and class-level inline documentation added.

Written by the indexing model from the issue text.