[Docs] Guide on how to write Flytekit shim plugins for Backend plugins

[cosmicBboy]

Description

Currently, we have docs for how to write backend plugins but don't have any guides for how to write a flytekit shim plugin that allows flytekit task authors to use that backend plugin.

An example of this is the Sagemaker Training plugin, which requires both.

This issue breaks up the tasks described in #1541

Context: https://www.youtube.com/watch?v=59w9HYOcJJ8&t=1282s

Are you sure this issue hasn't been raised already?

• Yes

Have you read the Code of Conduct?

• Yes

[github-actions]

Hello 👋, This issue has been inactive for over 9 months. To help maintain a clean and focused backlog, we'll be marking this issue as stale and will close the issue if we detect no activity in the next 7 days. Thank you for your contribution and understanding! 🙏

[github-actions]

Hello 👋, This issue has been inactive for over 9 months and hasn't received any updates since it was marked as stale. We'll be closing this issue for now, but if you believe this issue is still relevant, please feel free to reopen it. Thank you for your contribution and understanding! 🙏

[eapolinario]

Flyte agents are the blessed way of extending Flyte.