SUBSYSTEMS.md: migrate subsystem list from wiki

SUBSYSTEMS.md

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+# RIOT Subsystem Maintainers
+
+This file gives an overview about the maintainers of RIOT subsystems. Whenever
+you submit an issue or a Pull Request, please assign one of the maintainers of
+the affected modules as an assignee in the issue tracker. The maintainer takes
+care of either fixing/merging the issue/PR herself or re-assign the issue to a
+qualified developer.
+
+**Note**: A developer might maintain more than one subsystem and an issue might
+affect more than one module.
+
+# CPU
+## ESP32, ESP8266
+
+## MSP430
+
+## ARM7
+
+## ARM Cortex-M
+
+## Atmel AVR (Atmega)
+
+## Native
+
+# Hardware modules
+
+## Network devices
+### IEEE 802.15.4
+
+### BLE
+
+### LoRa
+
+### CC110X
+
+### ESP-NOW
+
+## Peripherals
+
+## Other drivers
+
+# Kernel (core)
+
+# System libraries
+
+## High-level timer API (xtimer, ztimer)
+
+## Power Management
+
+## File Systems
+
+## USB support
+
+## POSIX support
+
+## Scripting languages
+
+## RUST support
+
+## C++ support
+
+## Crypto
+
+# Networking
+
+## Network stacks
+### GNRC
+
+### CCN-Lite
+
+### OpenThread
+
+### LWIP
+
+### OpenWSN
+
+## Physical/Link Layer
+### LoRaWAN
+
+### BLE (nimble)
+
+### IEEE 802.15.4
+
+## Interfaces
+### Sock
+
+### Netif
+
+### Netdev
+
+## 6LowPAN
+
+## Application protocols
+### CoAP
+
+### MQTT
+
+### LWM2M
+
+## Others
+
+### DTLS
+
+### RPL
+
+# Build System
+
+# Documentation
+
+# Testing/CI