Improving the documentation regarding the setup of anti spam measure (#1404)

Added a new section for antispam features, moved the HCAPTCHA_SITE_KEY there and added more informations on how to enable this feature

content/en/admin/config.md

@@ -808,6 +808,13 @@ It is important to use a supported file format (JPEG or PNG, not SVG).
 
 ## Limits {#limits}
 
+### Anti Spam / Abuse 
+
+#### `HCAPTCHA_SITE_KEY`
+#### `HCAPTCHA_SECRET_KEY`
+
+If set, registrations confirm page will display a captcha, see [Captcha](https://docs.joinmastodon.org/admin/optional/captcha/)
+
 ### Email domains
 
 #### `EMAIL_DOMAIN_ALLOWLIST`
@@ -870,10 +877,6 @@ Defaults to `https://github.com/$GITHUB_REPOSITORY`
 
 #### `FFMPEG_BINARY`
 
-#### `HCAPTCHA_SITE_KEY`
-
-Set this to your hCaptcha site key to enable captchas on the account confirmation page using hCaptcha.
-
 Defaults to empty value (not enabled)
 
 #### `LOCAL_HTTPS`