Document customizing snippet extraction (#503)

Author: ErikSchierboom

building/tooling/snippet-extractor.md

@@ -3,9 +3,27 @@
 The [Snippet Extractor][snippet-extractor] takes a student's submission and extracts the first ten "interesting" lines of code from it.
 The extracted snippet is shown on various pages on the website.
 
-By default, the first ten lines are shown, ignore leading empty lines.
+By default, the first ten lines are shown, ignoring leading empty lines.
 These first ten lines _could_ include things like comments, which are usually not ideal to be presented in a snippet.
-Each track can [customize the snippet extraction][snippet-extractor-customize], for example to remove said comments.
+
+## Customizing snippet extraction
+
+We recommend each track [customizes snippet extraction][snippet-extractor-customize] to at least remove comments from the source code.
+To customize snippet extraction, you'll need to:
+
+1. Add a `lib/languages/<track_slug>.txt` file which configures snippet extraction.
+   This file can use either [basic mode][basic-mode] or [extended mode][extended-mode].
+   Most tracks use extended mode, as it gives more flexibility.
+2. Add one or more test cases to a `tests/<track_slug>/<test_slug>` directory.
+   Each test directory will need two files:
+
+   1. `tests/<track_slug>/<test_slug>/code.<track_extension>`: the code to run the snippet extractor on
+   2. `tests/<track_slug>/<test_slug>/expected_snippet.<track_extension>`: the expected snippet
+
+See [this example pull request][example-pr] that customizes snippet extraction for the Arturo language.
 
 [snippet-extractor]: https://github.com/exercism/snippet-extractor/
-[snippet-extractor-customize]: https://github.com/exercism/snippet-extractor/#add-your-language
+[snippet-extractor-customize]: https://github.com/exercism/snippet-extractor/?tab=readme-ov-file#customizing-snippet-extraction
+[example-pr]: https://github.com/exercism/snippet-extractor/pull/94
+[basic-mode]: https://github.com/exercism/snippet-extractor/blob/main/docs/basic.md
+[extended-mode]: https://github.com/exercism/snippet-extractor/blob/main/docs/extended.md