Modernize README to follow Ruby open source best practices (#77)

* docs: modernize README to follow Ruby open source best practices

- Add badges for gem version, CI status, and license
- Add table of contents for easier navigation
- Add Ruby compatibility matrix (3.1-3.4)
- Improve installation with Bundler instructions
- Use collapsible sections for GSL install and benchmarks
- Add syntax highlighting to all code blocks
- Add Development section with setup and test instructions
- Add Contributing guidelines
- Improve examples (spam filter, persistence, LSI search)
- Reorganize structure with Quick Start sections

Closes #76

* chore: update minitest to 6.0.1 for Ruby 4.0 support

* chore: drop Ruby 3.1 support (EOL March 2025)

* chore: add ostruct gem and update rake for Ruby 4.0

Author: cardmagic

.github/workflows/ruby.yml

@@ -33,7 +33,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby-version: ['3.1', '3.2', '3.3', '3.4']
+        ruby-version: ['3.2', '3.3', '3.4', '4.0']
 
     steps:
     - uses: actions/checkout@v4

Gemfile

@@ -4,6 +4,7 @@ gemspec
 gem 'fast-stemmer'
 gem 'matrix'
 gem 'mutex_m'
+gem 'ostruct'
 
 group :test do
   gem 'simplecov', require: false

Gemfile.lock

@@ -17,8 +17,10 @@ GEM
     language_server-protocol (3.17.0.5)
     lint_roller (1.1.0)
     matrix (0.4.2)
-    minitest (5.18.1)
+    minitest (6.0.1)
+      prism (~> 1.5)
     mutex_m (0.2.0)
+    ostruct (0.6.3)
     parallel (1.27.0)
     parser (3.3.10.0)
       ast (~> 2.4.1)
@@ -28,7 +30,7 @@ GEM
       stringio
     racc (1.8.1)
     rainbow (3.1.1)
-    rake (13.0.6)
+    rake (13.3.1)
     rdoc (6.5.1.1)
       psych (>= 4.0.0)
     regexp_parser (2.11.3)
@@ -74,6 +76,7 @@ DEPENDENCIES
   matrix
   minitest
   mutex_m
+  ostruct
   rdoc
   rubocop
   rubocop-minitest