Add documentation for installing WP-CLI packages

[Copilot]

Addresses #98 implementation by documenting how to install WP-CLI packages, including version constraint syntax for wp package install.

Changes

• New guide (guides/installing-packages.md): General guide for installing WP-CLI packages with sections on:

  • What WP-CLI packages are
  • How to install packages (by name, Git URL, local directory, .zip file)
  • Version constraints with common examples for exact versions, ranges (^, ~), stability flags (@stable), and development branches
  • Defers to official Composer documentation for detailed explanations on version constraints

• Updated guide index (guides.md): Added new guide to listing

Example Usage

# Install a package by name
wp package install wp-cli/server-command

# Install from a Git URL
wp package install https://github.com/wp-cli/server-command.git

# Install with version constraint
wp package install wp-cli/server-command:^2.0

# Install stable version
wp package install wp-cli/server-command:@stable

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.