Document in our manual how to create an external binding generator

[tarikeshaq]

With #1201 landed and 0.18.0 released we now support crates outside of uniffi-rs implementing their own binding generators, but we don't have a good document explaining how to do so.

#1201 and the ADR that introduced the support are good places to start.

Additionally, see https://github.com/bendk/gecko-dev/tree/uniffi/toolkit/components/uniffi-bindgen-gecko-js for an example (keep in mind that example is more complex than other binding generators due to it being catered to the Firefox JS engine)

┆Issue is synchronized with this Jira Task ┆friendlyId: UNIFFI-163