[spa77k]

Closes #740.

Adds a "Using your own channel" section to the Paper plugin messaging page:

• Using a custom channel (e.g. myplugin:main) instead of BungeeCord, with an example that registers it and sends and receives messages on it.
• A caution that Paper only sends a plugin message if the player's connection has registered the channel with minecraft:register, and otherwise silently drops it, as mentioned in the issue. It explains how the other end registers the channel: Velocity's ChannelRegistrar (linking to the Velocity plugin messaging page), BungeeCord's ProxyServer#registerChannel, or the client-side mod itself.
• A note that incoming messages need no such registration, so players can also send messages on the channel.

The behavior was checked against the source:

• Paper: CraftPlayer#sendPluginMessage (only sends if channels() contains the channel), the minecraft:register handling in ServerCommonPacketListenerImpl, and StandardMessenger#validateAndCorrectChannel.
• Velocity: ClientPlaySessionHandler#handleBackendJoinGame registers the ChannelRegistrar channels with the backend server.
• BungeeCord: ServerConnector sends registerChannels to the backend server.

Verification:

• pnpm build and prettier --check on the changed file passed, and the new jd: links resolve to existing Javadoc pages.

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