add anti-spam documentation

It describes how anti-spam works now and its future.

site/src/docs/manuals/spam/index.md

@@ -0,0 +1,18 @@
+---
+title: Anti-Spam
+---
+
+## How anti-spam works
+
+Basic real-time bot protection relies on simply testing if user sent invisible input form field and then login is rejected as it's known to be a bot.
+
+More sophisticated mechanism is discussed in the issue [#754](https://github.com/umputun/remark42/issues/754) but not yet implemented.
+
+## How `cleanup` anti-spam works
+
+During the `cleanup` command run, spam comments are detected and removed (if `--dry-run` parameter was not specified). Reaching score of 50 is considered as spam, and here are the scores:
+
+- 12.5 points for each occurrence of the provided bad words (so that 4 bad words will result in 50 points)
+- 10 points if user is in the provided list of bad users
+- 10 points if comment has any links, and another 10 if there are more than 5 of them
+- 20 points if comment score is 0 (e.g. nobody voted for it)

site/src/docs/nav.json

@@ -67,6 +67,10 @@
       {
         "title": "Gatsby Integration",
         "href": "/manuals/integration-with-gatsby/"
+      },
+      {
+        "title": "Anti-Spam",
+        "href": "/manuals/spam/"
       }
     ]
   },