[Docs] Document the Converter type and all the cases it needs to support

[czechboy0]

Motivation

I wrote down how the Converter works and all the cases we need to support. That'll help prepare ground for the refactor needed for #43.

Also created space for maintainer-focused docs.

Modifications

Added docs, which I'll use in a subsequent PR to ensure all the cases listed are implemented and tested.

Result

Converter now has higher level docs, and maintainers have a place to add more docs.

Test Plan

Previewed locally.

[simonjbeaumont]

Thanks for starting this document @czechboy0, it's great to get some of these maintainer-focused docs going.

I had some questions about whether we need to have the combinatorial explosion of methods that all need to be individually targeted by the generated code or whether we could manage to have something more general-purpose, which might farm out to other private things. If it doesn't have too much of an overhead, it might simplify the generated code and the generator a bit?