docs: add guide on handling unmatched formatters

Fixes #496

docs/content/.pages

@@ -1,4 +1,5 @@
 nav:
     - getting-started
+    - guides
     - contributing
-    - reference
+    - reference

docs/content/guides/unmatched-formatters.md

@@ -0,0 +1,47 @@
+# Handling Unmatched Files
+
+By default, treefmt lists all files that aren't matched by any formatter:
+
+```console
+$ treefmt
+WARN no formatter for path: .gitignore
+WARN no formatter for path: LICENSE
+WARN no formatter for path: README.md
+WARN no formatter for path: go.mod
+WARN no formatter for path: go.sum
+WARN no formatter for path: build/build.go
+# ...
+```
+
+This helps you decide whether to add formatters for specific files or ignore them entirely.
+
+## Customizing Notifications
+
+### Reducing Log Verbosity
+If you find the unmatched file warnings too noisy, you can lower the logging level in your config:
+
+`treefmt.toml`:
+```toml
+on-unmatched = "debug"
+```
+
+To later find out what files are unmatched, you can override this setting via the command line:
+```console
+$ treefmt --on-unmatched warn
+```
+
+### Enforcing Strict Matching
+Another stricter policy approach is to fail the run if any unmatched files are found.
+This can be paired with an `excludes` list to ignore specific files:
+
+`treefmt.toml`:
+```toml
+# Fail if any unmatched files are found
+on-unmatched = "fatal"
+
+# List files to explicitly ignore
+excludes = [
+  "LICENCE",
+  "go.sum",
+]
+```