Update gemspec for release, new metadata (mmistakes#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

just-the-docs.gemspec

@@ -2,20 +2,25 @@
 
 Gem::Specification.new do |spec|
   spec.name          = "just-the-docs"
-  spec.version       = "0.3.3"
-  spec.authors       = ["Patrick Marsceill"]
-  spec.email         = ["patrick.marsceill@gmail.com"]
+  spec.version       = "0.4.0.rc1"
+  spec.authors       = ["Patrick Marsceill", "Matthew Wang"]
+  spec.email         = ["patrick.marsceill@gmail.com", "matt@matthewwang.me"]
 
   spec.summary       = %q{A modern, highly customizable, and responsive Jekyll theme for documention with built-in search.}
   spec.homepage      = "https://github.com/just-the-docs/just-the-docs"
   spec.license       = "MIT"
+  spec.metadata      = {
+    "bug_tracker_uri"   => "https://github.com/just-the-docs/just-the-docs/issues",
+    "changelog_uri"     => "https://github.com/just-the-docs/just-the-docs/blob/main/CHANGELOG.md",
+    "documentation_uri" => "https://just-the-docs.github.io/just-the-docs/",
+    "source_code_uri"   => "https://github.com/just-the-docs/just-the-docs",
+  }
 
-  spec.files         = `git ls-files -z`.split("\x0").select { |f| f.match(%r{^(assets|bin|_layouts|_includes|lib|Rakefile|_sass|LICENSE|README)}i) }
+  spec.files         = `git ls-files -z`.split("\x0").select { |f| f.match(%r{^(assets|bin|_layouts|_includes|lib|Rakefile|_sass|LICENSE|README|CHANGELOG|favicon)}i) }
   spec.executables   << 'just-the-docs'
 
   spec.add_development_dependency "bundler", "~> 2.3.5"
   spec.add_runtime_dependency "jekyll", ">= 3.8.5"
   spec.add_runtime_dependency "jekyll-seo-tag", ">= 2.0"
   spec.add_runtime_dependency "rake", ">= 12.3.1"
-
 end