[Doc] Update homepage with just-the-docs theme

Follow-up to https://github.com/modelcontextprotocol/ruby-sdk/pull/181.

The landing page at https://ruby.sdk.modelcontextprotocol.io previously only listed version links
to RubyDoc.info. Add multi-page documentation using the just-the-docs Jekyll theme with
sidebar navigation, search, and code highlighting.
Pages: Introduction, Installation, Building Servers, and Building Clients.

This PR introduces a basic set of pages. Additional content can be added incrementally in future updates.

docs/installation.md

@@ -0,0 +1,32 @@
+---
+layout: default
+title: Installation
+nav_order: 2
+---
+
+# Installation
+
+Add this line to your application's Gemfile:
+
+```ruby
+gem 'mcp'
+```
+
+And then execute:
+
+```console
+$ bundle install
+```
+
+Or install it yourself as:
+
+```console
+$ gem install mcp
+```
+
+You may need to add additional dependencies depending on which features you wish to access. For example, the HTTP client transport requires the `faraday` gem:
+
+```ruby
+gem 'mcp'
+gem 'faraday', '>= 2.0'
+```