Clarify that Context types are defined in JSON and may be handled differently in specific API bindings

[kriswest]

Enhancement Request

As identified in the discussion on #1479, the Context Part of the FDC3 standard defines the JSON encoding of Context objects, which is (in JS and .NET so far) translated into a set of types defined for that language, which may change the type of specific fields to an idiomatic type for that language (for example Date fields in JSON are string encoded according to ISO 8601-1:2019, while in JavaScript/TypeScript the Date type is used, which stringifies to the same format).

The Context overview should:

• include content explaining the difference between the JSON definitions and definitions for a specific languages API binding.
• include language-specific details on field type conventions (if they differ by language binding - e.g. dates and times).
• link to source code and packages for language-specific bindings for context types.

@hellosmithy @bingenito @kemerava