Add initial Validation library documentation with Hugo preview infrastructure

[Copilot]

This PR establishes the documentation foundation for the Validation library, including a Hugo-based preview system, working code examples, and initial user-facing content.

Main code changes

• Gradle plugin auto-applies dependencies: Validation plugin now automatically applies Protobuf and Compiler Gradle plugins, eliminating manual configuration
• JVM runtime API exposure: Changed Base.lib from implementation to api scope in jvm-runtime module
• Build system: Added TIMEOUT_MINUTES property for long-running CI tasks, bumped Gradle to 9.3.1

Documentation infrastructure

• Hugo integration: Created docs/ as first-class Gradle module with Go module support and Hugo theme integration
• Build automation:
  • UpdatePluginVersion task maintains version consistency across documentation examples
  • Scripts for building, serving, and embedding live code samples from _code/ projects
  • GitHub workflow validates embedded code references
• Example projects:
  • first-model: Basic validation with BankCard message demonstrating (required) and (pattern) constraints
  • first-model-with-framework: Validation integrated with Spine framework
  • Both include Java and Kotlin test examples

Documentation content

• Getting Started:
  • Adding Validation to Gradle builds with plugin configuration
  • First validated model tutorial using BankCard example with regex patterns for card numbers and owner names
• Introduction: Philosophy and target audience sections with sentence-case formatting
• Validation concepts: Component architecture and constraint organization

Example from the first model guide:

message BankCard {
    string digits = 1 [
        (required) = true,
        (pattern).regex = "^\\d{4} \\d{4} \\d{4} \\d{4}$"
    ];
    
    string owner = 2 [
        (required) = true,
        (pattern).regex = "^(?=.{4,})[A-Z][A-Za-z]*(?: [A-Za-z]+)*$"
    ];
}

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[alexander-yevsyukov]

Closing as auxiliary to #261.