Add exec package reference documentation

[Copilot]

This PR adds comprehensive reference documentation for the exec package to the gosoline documentation site. The documentation follows the established pattern used by other package reference docs and provides detailed coverage of the package's retry and backoff mechanisms.

What's Added

The new documentation includes:

Core Components

• Executor interface and implementations (BackoffExecutor, DefaultExecutor)
• Error classification system with built-in checkers for network/connection errors
• Context utilities for delayed cancellation and deadline management (WithDelayedCancelContext, WithStoppableDeadlineContext, etc.)
• Configuration system with predefined backoff types

Documentation Structure

• Package overview explaining retry logic and resilience patterns
• Detailed function documentation with usage examples and GitHub links
• Interface definitions with descriptions
• Configuration options and settings table
• Predefined backoff types: "api", "once", "infinite"
• Three comprehensive usage examples:
  • Basic HTTP client with retries
  • Database operations with custom error handling
  • Configuration-based executor setup

Key Features Documented

• Exponential backoff with intelligent error classification
• Built-in error checkers for common network issues (timeouts, connection errors, TLS handshake failures)
• Resource tracking and logging capabilities
• Configuration inheritance and fallback mechanisms
• Context management for graceful operation cancellation

The documentation provides practical examples showing how developers can use the exec package to make their applications more resilient to transient failures in distributed systems.

Fixes #1265.

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