Add documentation for joining Discord servers via linked lobbies (#7810)

Documents the JoinLinkedLobbyGuild functionality that allows players
to join Discord servers directly from game lobbies. Includes
platform-specific considerations and security warnings for server
administrators.

Author: markmandel

docs/discord-social-sdk/development-guides/linked-channels.mdx

@@ -293,6 +293,67 @@ int main() {
 }
 ```
 
+## Joining Discord Servers via Linked Lobbies
+
+Once a lobby is linked to a Discord channel, players can join the associated Discord server directly from your game. This feature simplifies the process of getting players into your Discord community by generating invites on-demand, eliminating the need to manually share invite links.
+
+The [`Client::JoinLinkedLobbyGuild`] function generates a one-time-use invite for the current user and on supported
+platforms, automatically navigates them to Discord to accept it.
+
+For example, an in-game player flow could look like:
+
+1. A Discord server admin links a channel to your game's lobby
+2. Players in the lobby see an option to "Join Discord Server" in your game
+3. When clicked, the SDK generates a unique invite and opens Discord
+4. The player accepts the invite and becomes a Discord server member
+
+:::info
+Only players with linked Discord accounts can join the server. If a player is using a provisional account, you should prompt them to link their Discord account first.
+:::
+
+```cpp
+const uint64_t lobbyId = 1234567890;
+
+// Invite the user to join the Discord guild associated with the linked lobby
+client->JoinLinkedLobbyGuild(
+    lobbyId,
+    // This is triggered when the user is using a provisional account, since
+    // the user needs a real Discord account to join the Discord server, so you don't need
+    // to implement a provisional user check on implementation.
+    [] {
+        // Show a message in your UI explaining that they need to link their Discord account
+        // and/or step them through the process
+        std::cout << "📝 User needs to link their Discord account\n";
+    },
+    // Called after the invite generation attempt completes, providing either a
+    // successful result with the invite URL or an error if the operation failed.
+    [](const discordpp::ClientResult &result, const std::string& inviteUrl) {
+        if(result.Successful()) {
+            std::cout << "✅ Discord invite generated successfully!\n";
+
+            // On console platforms, you'll need to display the invite URL
+            // for users to manually navigate to
+            #ifdef CONSOLE_PLATFORM
+            std::cout << "Join the Discord server at: " << inviteUrl << "\n";
+            // Display this URL in your game's UI for the player to use
+            #endif
+        } else {
+            std::cerr << "❌ Failed to generate Discord invite\n";
+        }
+    }
+);
+```
+
+### Platform Considerations
+
+- **Desktop**: The SDK automatically opens the Discord client or web browser with the invite
+- **Console**: Since console platforms cannot navigate to Discord directly, you should display the invite URL in your game's UI so players can use it on another device
+
+:::warn
+Discord server admins cannot restrict who can join the server via this method. Any player in a linked lobby can generate an invitation to the server, regardless of their lobby permissions.
+<br/>Make sure to make your players aware to only link channels in servers you trust your players to join, and/or provide in-game options to disable this feature for certain lobbies.
+:::
+
 ---
 
 ## Next Steps
@@ -323,5 +384,6 @@ int main() {
 {/* Autogenerated Reference Links */}
 [`Client::GetGuildChannels`]: https://discord.com/developers/docs/social-sdk/classdiscordpp_1_1Client.html#adba1e5a83c219a9c4f6dab1657778017
 [`Client::GetUserGuilds`]: https://discord.com/developers/docs/social-sdk/classdiscordpp_1_1Client.html#aac1ec02df6074ed9213ce230e6a42fe1
+[`Client::JoinLinkedLobbyGuild`]: https://discord.com/developers/docs/social-sdk/classdiscordpp_1_1Client.html#a54ec764e72e168de419ac14e24e8fc60
 [`Client::LinkChannelToLobby`]: https://discord.com/developers/docs/social-sdk/classdiscordpp_1_1Client.html#a3114d58d50d4d2cb5752d95e121315d4
 [`Client::UnlinkChannelFromLobby`]: https://discord.com/developers/docs/social-sdk/classdiscordpp_1_1Client.html#a28f78a6fe46eb11eb54ee9b53fa94ffe