Add localhost as default external_link

Author: reakaleek

src/Elastic.Markdown/IO/ConfigurationFile.cs

@@ -23,7 +23,7 @@ public record ConfigurationFile : DocumentationFile
 	public HashSet<string> Files { get; } = new(StringComparer.OrdinalIgnoreCase);
 	public HashSet<string> ImplicitFolders { get; } = new(StringComparer.OrdinalIgnoreCase);
 	public Glob[] Globs { get; } = [];
-	public HashSet<string> ExternalLinkHosts { get; } = new(StringComparer.OrdinalIgnoreCase) { "elastic.co", "github.com", };
+	public HashSet<string> ExternalLinkHosts { get; } = new(StringComparer.OrdinalIgnoreCase) { "elastic.co", "github.com", "localhost", };
 
 	private readonly Dictionary<string, string> _substitutions = new(StringComparer.OrdinalIgnoreCase);
 	public IReadOnlyDictionary<string, string> Substitutions => _substitutions;