[doc] Best Practices Document

Nobody has claimed this yet.

When using Lit, it could be helpful to have a documented list of best practices to reference.
Maybe in the form of a style-guide? E.g., https://google.github.io/styleguide/tsguide.html

This way when user questions come up asking for a best practice to do something – we can document it for all.

Rough thoughts:
- Don't reach across shadow roots. (And if you need to, how can you? – properties/context down, events up)
- Mimic native element behavior. LitElements are used just like native HTML elements, make them behave similarly. (elaborate).
- `render()` should be a pure function of state and not have side effects.

Start by reviewing the proposed topics in this issue and the existing Lit documentation on the website. Define the scope and structure of a best-practices or style-guide document, then consider the work done when the agreed guidance is published and covers the selected topics.

Written by the indexing model from the issue text.