Add page on creation of derived datasets and models

[tompollard]

We should add a page to the website to outline acceptable approaches for creating derived datasets and models. For example,

• What are the rules on distributing derived datasets and models?
• What are our preferred naming conventions for derived datasets?

[tompollard]

I'd meant to do this via pull request but ended up pushing directly to main (sorry!).

Page added in: 2548c24

The page is available at: https://mimic.mit.edu/docs/community/derived/