[#10,!3] Added 4 missing files for ha-* commands.

doc/api/ha-continue.json

+{
+    "name": "ha-continue",
+    "brief": "This command is used to resume the operation of the paused HA state machine.",
+    "description": "See <xref linkend=\"anchor-in-the-docs\"/>",
+    "support": [ "kea-dhcp4", "kea-dhcp6" ],
+    "avail": "1.4.0",
+    "hook": "high_availability"
+}

doc/api/ha-heartbeat.json

+{
+    "name": "ha-heartbeat",
+    "brief": "This command is sent internally by Kea partner when operating
+              in High Availability (HA) mode. It should not be used by users,
+              unless you want to implement complete HA replacement.",
+    "description": "See <xref linkend=\"ha-server-states\"/>",
+    "support": [ "kea-dhcp4", "kea-dhcp6" ],
+    "avail": "1.4.0",
+    "hook": "high_availability",
+    "cmd-syntax": "tbd",
+    "resp-syntax": "tbd"
+}

doc/api/ha-scopes.json

+{
+    "name": "ha-scopes",
+    "brief": "This command allows for modifying the High Availability (HA) scopes
+              that the server is serving.a sentence or two explaining what this command does",
+    "description": "See <xref linkend=\"command-ha-scopes\"/>",
+    "support": [ "kea-dhcp4", "kea-dhcp6" ],
+    "avail": "1.4.0",
+    "hook": "high_availability",
+    "cmd-syntax": "{
+    \"command\": \"ha-scopes\",
+    \"service\": [ <service, typically \"dhcp4\" or \"dhcp6\"> ],
+    \"arguments\": {
+        \"scopes\": [ \"HA_server1\", \"HA_server2\" ]
+    }
+}
+"
+}

doc/api/ha-sync.json

+{
+    "name": "ha-sync",
+    "brief": "The command is issued to instruct the server running in HA mode to 
+              synchronize its local lease database with the selected peer.",
+    "description": "See <xref linkend=\"command-ha-sync\"/>",
+    "support": [ "kea-dhcp4", "kea-dhcp6" ],
+    "avail": "1.4.0",
+    "hook": "high_availability",
+    "cmd-syntax": "{
+    \"command\": \"ha-sync\",
+    \"service\": [ <service affected: \"dhcp4\" or \"dhcp6\" ],
+    \"arguments\": {
+        \"server-name\": <name of the partner server>,
+        \"max-period\": <integer, in seconds>
+    }
+}"
+}