Add documentation for expermental NewJoinerProtection. (#26)

docs/protections/new-joiner-protection.md

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+---
+sidebar_label: New Joiner Protection
+---
+
+<!--
+SPDX-FileCopyrightText: 2024 Gnuxie <[email protected]>
+
+SPDX-License-Identifier: CC-BY-SA-4.0
+-->
+
+# New Joiner Protection
+
+:::danger
+
+This is an experimental protection that should only be used as a last
+resort when your community is under persistent attack.
+
+This protection is unavailable in Draupnir version `2.0.0-beta.4` and
+below.
+
+This protection may be removed or changed entirely.
+
+:::
+
+This is a protection that can help when one of your rooms is coming
+under a persistent attack from new joiners. Whether that be wave spam
+or targeted attacked.
+
+The protection works by banning new users from a set of configured
+servers, leaving existing community members from the target servers
+unaffected. The protection serves as an alternative to banning
+the server outright.
+
+## Configuration
+
+To configure the protection, append the following under the
+`protections:` header in your config file:
+
+```yaml
+newJoinerProtection:
+  serverNames:
+    - "example.com"
+```
+
+You will need to restart the bot to make this effective. We recommend
+that you leave servers where abuse typically originates in the
+configuration file. Then the protection can be enabled and disabled at
+will without restarting the bot.
+
+The ban message can also be changed with the option `banMessage:`.
+
+By default this reads "Unfortunately we cannot accept new users from
+your homeserver at this time.".