document embedding API guidelines

[jdm]

servo/servo#43617 (comment)

[jdm]

We should document guidelines about re-exporting types from servo crates as part of libservo's API so embedders only need to depend on the libservo crate.

[jdm]

servo/servo#44343 (comment)

[jdm]

Document restrictions against exposing testing-only API methods and alternatives: pref-gated webidl methods that can be invoked via JS evaluation instead.