Ignore /docs folder.

This will make the WordPress plugin review team happy. Calling files remotely:

```
pronamic-pay-with-mollie-for-woocommerce/packages/pronamic/wp-money/docs/hooks.md:33:<p align="center"><a href="https://github.com/pronamic/wp-documentor"><img src="https://cdn.jsdelivr.net/gh/pronamic/wp-documentor@main/logos/pronamic-wp-documentor.svgo-min.svg" alt="Pronamic WordPre [...]
```

pronamic/wp-pay-core#143

.gitattributes

@@ -1,6 +1,7 @@
 /.gitattributes    export-ignore
 /.github/          export-ignore
 /.gitignore        export-ignore
+/docs              export-ignore
 /Gruntfile.js      export-ignore
 /phpcs.xml.dist    export-ignore
 /phpmd.ruleset.xml export-ignore