docs: Add documentation of the ports directory

Author: iqyx

ports/index.rst

@@ -2,6 +2,23 @@
 Ports
 ===========================================
 
+A "port" is the board‑/hardware‑specific implementation layer that adapts the generic plumCore framework
+to run on a particular microcontroller board or hardware platform.
+
+It contains, among others:
+
+- clock initialization happening in the early stage of the boot process
+- configuration of GPIO ports
+- instantiation of services which are run on the target hardware and injecting the required dependencies
+- advertising interfaces of the instantiated services
+- interrupt to service glue for interrupts
+- hardware revision detecting logic for cases when a single firmware runs on hardware of
+  multiple (semi-)compatible revisions
+- linker script for the target MCU
+- FreeRTOS configuration specific for the target MCU
+- SConscript to build the port code
+- documentation of the port in the doxygen form (for the code) and reST (well, for the rest and high-level documents)
+
 .. toctree::
 
 	nwdaq-br28-fdc/index.rst