Enable setting custom GitHub Host for GitHub MCP

[dongdong867]

This pull request adds support for specifying a custom GitHub host URL, which is useful for users working with GitHub Enterprise Server or custom GitHub deployments.
The changes update configuration options, documentation, and environment variable handling to accommodate the new github_host setting.

Support for custom GitHub host:

• Added an optional github_host field to the GitHubContextServerSettings struct in src/mcp_server_github.rs to allow specifying a custom GitHub URL.
• Updated the environment variable setup in the GitHubModelContextExtension implementation to include GITHUB_HOST when github_host is provided and non-empty.

Configuration and documentation updates:

• Added commented example for github_host in configuration/default_settings.jsonc to guide users on how to specify a custom GitHub host.
• Updated configuration/installation_instructions.md to document the new github_host option and provide a link to further instructions for GitHub Enterprise Server.

[jeffreyguenther]

Taking a look now.