Add moderation and spam control explanation (#624)

docs/explanation/moderation-and-spam-control.md

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+# Moderation and spam control
+
+Moderation and spam control are critical to maintaining the health and security
+of any communication platform, and Synapse is no exception. As a Matrix
+homeserver, Synapse enables decentralized communication, which brings challenges
+in combating abuse and ensuring user safety. This document
+explores charm configurations and tools (Synapse Invite Checker module and Mjolnir)
+that can help set moderation and spam control in Synapse.
+
+---
+
+## Charm configurations
+
+Synapse charm provide administrators with configuration options to fine-tune
+moderation and spam control. Below are some key settings:
+
+### [`rc_joins_remote_burst_count`](https://charmhub.io/synapse/configurations#rc_joins_remote_burst_count)
+
+Limits the number of remote rooms a user can join before being throttled.
+
+- Default: `10`
+
+### [`rc_joins_remote_per_second`](https://charmhub.io/synapse/configurations#rc_joins_remote_per_second)
+
+Defines the rate limit for how many remote rooms a user can join per second.
+
+- Default: `0.01`
+
+Refer to [Ratelimiting](https://element-hq.github.io/synapse/latest/usage/configuration/config_documentation.html#ratelimiting) in Synapse configuration documentation for more details.
+
+### [`block_non_admin_invites`](https://charmhub.io/synapse/configurations#block_non_admin_invites)
+When enabled, this configuration blocks room invites to users on the server,
+except for those sent by local server admins.
+- Default: `false`
+
+Refer to [block_non_admin_invites](https://element-hq.github.io/synapse/latest/usage/configuration/config_documentation.html#block_non_admin_invites) in Synapse configuration documentation for more details.
+
+---
+
+## Synapse Invite Checker module
+
+The **Synapse Invite Checker** module provides additional flexibility in
+controlling spam and abuse by monitoring and filtering room invites. This tool
+leverages allowlists and blocklists to ensure that only trusted sources can
+send invites.
+
+### Configurations
+
+#### [`invite_checker_blocklist_allowlist_url`](https://charmhub.io/synapse/configurations#invite_checker_blocklist_allowlist_url)
+URL to fetch a JSON file containing the allowlist and blocklist.
+
+#### [`invite_checker_policy_rooms`](https://charmhub.io/synapse/configurations#invite_checker_policy_rooms)
+A comma-separated list of rooms used by the invite checker module to enforce
+policies.
+
+### More information
+For details and implementation, visit the module’s repository: [Synapse Invite Checker](https://git.buechner.me/nbuechner/synapse-invite-checker).
+
+---
+
+## Mjolnir
+
+Synapse charm also has Mjolnir in place. Mjolnir is an all-in-one moderation
+tool designed to protect Synapse server from malicious invites, spam messages,
+and other unwanted activities.
+
+### Key features
+
+- Bans and redactions: Quickly remove malicious users and their messages from
+rooms.
+- Anti-spam: Automatically detect and mitigate spam activity.
+- Server ACLs: Manage and enforce access control lists at the server level.
+- Room directory changes and alias transfers: Adjust room visibility and manage
+aliases efficiently.
+- Account deactivation: Disable abusive or compromised accounts.
+- Room shutdown: Close problematic rooms completely.
+
+### More information
+
+For more details and implementation guidance, refer to the [Mjolnir GitHub repository](https://github.com/matrix-org/mjolnir).
+