Update gemspec for release, new metadata (#911)

A couple of changes to prep for v0.4.0-rc1:

- bump the version
- add myself to the authors + email (this is primarily so that Patrick doesn't get siloed as the only maintainer; if you'd like to be added @just-the-docs/maintainers, let me know!)
- adds some more metadata ([gemspec docs](https://guides.rubygems.org/specification-reference/#metadata))
- updated the path to also include the CHANGELOG and favicon in the gem bundle