Document how developers can implement their own "strategy"

[adamsachs]

Docs Update Description

Once #1254 is merged, developers outside of the core fidesops repo will be able to implement their own AuthenticationStrategy, PaginationStrategy or PostProcessorStrategy, similar to how they can already implement their own MaskingStrategy. It would be nice to provide some formal documentation on how this can be done.

Additional context

The PR description of #1254 contains some notes about how strategies can be implemented - but more detail can be provided if needed.

Note that our documentation already includes an Extensibility section in its masking strategy tutorial - once the above PR is merged, the process should look very similar for the other strategy types mentioned above.