Add a sentence on how to fix the external URI warning (#142)

Closes #140

## Changes

Add a sentence on how to fix the warning, explaining where to add the
base domain.

```shell
Warning: External URI 'https://docs.elastic.dev/migration/site-level/content#asciidoctor-conf-yml' is not allowed. Add 'elastic.dev' to the 'external_hosts' list in /Users/jcal/Projects/docs-builder/docs/source/docset.yml to allow links to this domain.
   ┌─[/Users/jcal/Projects/docs-builder/docs/source/testing/index.md]
   │
   │
   └─
```

## Notes

The warning is missing the actual markdown line where the error is
happening. (See example from
#140)
I saw the same behavior with @Mpdreamz when he demoed the app to me.

src/Elastic.Markdown/Myst/InlineParsers/DiagnosticLinkInlineParser.cs

@@ -62,7 +62,15 @@ public override bool Match(InlineProcessor processor, ref StringSlice slice)
 		{
 			var baseDomain = string.Join('.', uri.Host.Split('.')[^2..]);
 			if (!context.Configuration.ExternalLinkHosts.Contains(baseDomain))
-				processor.EmitWarning(line, column, length, $"external URI: {uri} ");
+			{
+				processor.EmitWarning(
+					line,
+					column,
+					length,
+					$"External URI '{uri}' is not allowed. Add '{baseDomain}' to the " +
+					$"'external_hosts' list in {context.Configuration.SourceFile} to " +
+					"allow links to this domain.");
+			}
 			return match;
 		}