feat: Add optional GitHub repository link to site configuration

[mhoshdev]

Why

To help users easily find the source code and encourage them to contribute back when they are using a hosted instance of the tool.

Description

Introduce an optional githubUrl field to the siteConfig. If this URL is provided, a GitHub icon and link should be displayed prominently in the header or footer, seamlessly connecting the feature board to its source repository.

Proposed Changes

• Update the siteConfig interface/type definition to include an optional githubUrl field.
• Modify the Header or Footer components to conditionally render a GitHub icon/link if the URL is present.
• Update documentation to explain how to enable this feature.