add custom overlays docs (#551)

docs/participate/network-maintenance/custom-overlays.md

@@ -0,0 +1,94 @@
+# Custom overlays
+
+TON `v2024.04` update introduces the ability to use custom overlays.  
+Currently they can be used only for broadcasting external messages. The main idea is to create private overlay with 
+sender nodes and validators. Only sender nodes can create broadcasts with external messages which will be (hopefully) 
+received by block collator and get into the block.
+
+## Default custom overlays
+
+Mytonctrl uses default custom overlays available at https://ton-blockchain.github.io/fallback_custom_overlays.json. To 
+stop participation in default custom overlays run commands
+```bash
+MyTonCtrl> set useDefaultCustomOverlays false
+MyTonCtrl> delete_custom_overlay default
+```
+
+## Create custom overlay
+
+### Collect adnl addresses
+
+To add validators to a custom overlay you can use either their `fullnode adnl id` available with `validator-console -c getconfig` 
+or `validator adnl id` which can be found in mytonctrl's status.
+To add liteservers to a custom overlay you must use their `fullnode adnl id`.
+
+### Create a config file
+
+Create a config file in format:
+
+```json
+{
+    "adnl_address_hex_1": {
+        "msg_sender": true,
+        "msg_sender_priority": 1
+    },
+    "adnl_address_hex_2": {
+        "msg_sender": false
+    },
+  ...
+}
+```
+
+`msg_sender_priority` determines the order of external message inclusion to blocks: first processed messages from higher priority source. Messages from public overlay and local LS have priority 0.
+
+**Note, all nodes listed in config should participate in overlay (in other words they need to add overlay with exactly this config), otherwise connectivity will be poor and broadcasts will fail**
+
+There is special word `@validators` to create a dynamic custom overlay that mytonctrl will generate automatically
+each round adding all current validators.
+
+### Add custom overlay
+
+Use mytonctrl command to add custom overlay:
+
+```bash
+MyTonCtrl> add_custom_overlay <name> <path_to_config>
+```
+
+Note, that name and config file **must** be the same on all overlay members. Check that overlay has been created using 
+mytonctrl `list_custom_overlays` command.
+
+### Debug
+
+You can set node verbosity level equals to 4 and grep logs with "CustomOverlay" word.
+
+## Delete custom overlay
+
+To remove custom overlay from a node, use mytonctrl command `delete_custom_overlay <name>`. 
+If the overlay is dynamic (i.e. there is `@validators` word in config) it will be deleted within one minute, otherwise it will be removed instantly. 
+To make sure that node has deleted custom overlay check `list_custom_overlays` mytonctrl and `showcustomoverlays` validator-console commands.
+
+## Low level
+
+List of validator-console commands to work with custom overlays:
+
+* `addcustomoverlay <path_to_config>` - add custom overlay to local node. Note, that this config  must be in a format other than config for mytonctrl:
+    ```json
+    {
+      "name": "OverlayName",
+      "nodes": [
+        {
+          "adnl_id": "adnl_address_b64_1",
+          "msg_sender": true,
+          "msg_sender_priority": 1
+        },
+        {
+          "adnl_id": "adnl_address_b64_2",
+          "msg_sender": false
+        }, ...
+      ]
+    }
+    ```
+* `delcustomoverlay <name>` - delete custom overlay from node.
+* `showcustomoverlays` - show list of custom overlays node knows about.
+
+

sidebars.js

@@ -641,6 +641,7 @@ const sidebars = {
         'participate/network-maintenance/nominators',
         'participate/network-maintenance/persistent-states',
         'participate/nodes/collators',
+        'participate/network-maintenance/custom-overlays',
       ],
     },
     {