The following commit rewrites the ESP-MESH docs. This commit includes
new diagrams and a structure approach to the mesh docs. Some mesh api references
have also been modified.
1. modify parent switch mechanism.
2. fix non-root doesn't clear the layer value in nvs.
3. fix retransmit the remove announcement packets.
4. add API esp_mesh_flush_upstream_packets().
5. automatically adjust passsive scan time based on the change of beacon interval.
1. add APIs to set/get announce interval values.
2. add API to enable/disable the min rate to 6Mbps for data packages.
3. add APIs to set/get RSSI threshold values.
4. fix ap_loss.
1. add mesh_assoc_t to esp_mesh_internal.h file.
2. rename "node" to "device" in esp_mesh.h.
3. add MESH_EVENT_SCAN_DONE event.
4. add APIs esp_mesh_scan_get_ap_record() and esp_mesh_scan_get_ap_ie_len() to get scan results.
5. modify API esp_mesh_set_self_organized() by adding parameter "select_parent".
6. modify API esp_mesh_set_parent() by adding parameter "parent_mesh_id"
7. add manual networking example.
