chore: add documentation uri

Author: yoshoku

hnswlib.gemspec

@@ -16,6 +16,7 @@ Gem::Specification.new do |spec|
   spec.metadata['homepage_uri'] = spec.homepage
   spec.metadata['source_code_uri'] = spec.homepage
   spec.metadata['changelog_uri'] = 'https://github.com/yoshoku/hnswlib.rb/blob/main/CHANGELOG.md'
+  spec.metadata['documentation_uri'] = 'https://yoshoku.github.io/hnswlib.rb/doc/'
   spec.metadata['rubygems_mfa_required'] = 'true'
 
   # Specify which files should be added to the gem when it is released.