Add custom source linker support for HTML output

[NicolasRouquette]

This PR introduces an optional SourceLinkerFn parameter to htmlOutputResults and htmlOutput, enabling custom source linking logic instead of relying solely on the default URL-based linker.

Motivation

The current doc-gen4 implementation computes source links internally using a fixed algorithm based on sourceUrl?. While this works well for standard GitHub repositories, some use cases require more sophisticated source linking:

• Multi-repository documentation - Projects that aggregate documentation from multiple repositories need different base URLs for different modules
• Custom hosting platforms - Source code hosted on platforms with non-standard URL patterns
• Monorepo support - Projects where different modules map to different subdirectories
• Integration with other tools - I'm developing a doc-gen4 "plugin" that combines doc-gen4 with additional documentation where I need to customize source URL generation.

New type alias:

abbrev SourceLinkerFn := Option String → Name → Option DeclarationRange → String

Updated signatures:

def htmlOutputResults (baseConfig : SiteBaseContext) (result : AnalyzerResult) 
    (sourceUrl? : Option String) (sourceLinker? : Option SourceLinkerFn := none) : IO (Array System.FilePath)

def htmlOutput (buildDir : System.FilePath) (result : AnalyzerResult) (hierarchy : Hierarchy)
    (sourceUrl? : Option String) (sourceLinker? : Option SourceLinkerFn := none) : IO Unit

Backward Compatibility

• The new sourceLinker? parameter defaults to none, preserving existing behavior
• When sourceLinker? is none, the existing SourceLinker.sourceLinker is used
• All existing code calling these functions without the new parameter continues to work unchanged

Example usage:

-- Custom source linker that handles multiple repositories
def mySourceLinker : SourceLinkerFn := fun _sourceUrl? module range =>
  let root := module.getRoot
  let baseUrl := if root == `MyLib then "https://github.com/org/mylib" 
                 else "https://github.com/org/other"
  -- ... compute URL with line range
  s!"{baseUrl}/blob/main/{path}#L{startLine}-L{endLine}"

-- Use custom linker
DocGen4.htmlOutputResults baseConfig result none (some mySourceLinker)