[Core Feature] Capture and save documentation from flytekit entities

[katrogan]

Motivation: Why do you think this is important?
See #531 for a richer discussion on this feature. For this specific issue, we will want to capture user documentation, links to source code and commit version and pipe that through the Flyte system and make entity descriptions for tasks, workflows, launch plans and executions retrievable via a separate endpoint.

Goal: What should the final outcome look like, ideally?
It should be possible to add RST-formatted documentation inline with flytekit entity definitions or when launching executions from any means and have that information made retrievable by end users.

Describe alternatives you've considered
Again, see #531

[Optional] Propose: Link/Inline OR Additional context
Design doc: https://docs.google.com/document/d/1wvCvM6pLQ17Zr0DcpS9es2vOFQsNeOaNN99ufGw8O1U/edit?usp=sharing

[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! 🙏

[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 engage on it to decide if it is still applicable.
Thank you for your contribution and understanding! 🙏