[DOC] document algorithms and in-process provider behavior

[toddbaert]

There's been some interested in people developing "in-process" flagd providers, that use json-logic and the flagd sync schema to basically do flag evaluations as flagd does, but directly in the client process.

There's not much to be done about this in this repo besides documentation. At a minimum, we should define:

• initialization, shutdown, error behavior - basically the life-cycle of the provider, similar to the aforementioned RPC example
  (a mermaid state diagram outline will likely be helpful here)
• configuration parameters and defaults
• detailed descriptions of how to implement all the custom evaluators (fractional evaluation, sem-ver, ends-with, starts-with)