Update docs for plugins

[jeffcharles]

Description of the change

Updates documentation with new project architecture and how to extend Javy with plugins.

Why am I making this change?

Part of #768. The documentation was a bit out-of-date.

Checklist

• I've updated the relevant CHANGELOG files if necessary. Changes to javy-cli and javy-plugin do not require updating CHANGELOG files.
• I've updated the relevant crate versions if necessary. Versioning policy for library crates
• I've updated documentation including crate documentation if necessary.