[docs] Message Forwards feature

[lytefast]

Add documentation for message forwards.

A message forward gives users the ability to send a snapshot of a message in one channel to another channel.

• snapshots are immutable, and do not receive updates from the original message (unlike replies)
• similar to a REPLY message, a forward is created using a message_reference

DDevs #api-announcement - thread there

Examples

Creates

{
    "message_reference": {
        "type": 1,  // FORWARD
        "message_id": 123456789012345678,
        "channel_id": 123456789012345678,
        "guild_id": 123456789012345678
    },
}

Responses

{
    "author": {
        "id": 123456789012345678,
    },
    "message_reference": {
        "type": 1,  // FORWARD
        "message_id": 123456789012345678,
        "channel_id": 123456789012345678,
        "guild_id": 123456789012345678
    },
    "message_snapshot": {
        "content": "hello world",
        "embeds": [
            // Embed objects
        ],
        "attachments": [
            // Attachment objects
        ],
    }
}