doc: massive refactor and simplification to migrate to mkdocs

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

doc/watchdog.md

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+Bundled Watchdog Daemon
+=======================
+
+When built `--with-watchdog` a separate service is built and installed
+in `/libexec/finit/watchdogd`.  If this exists at runtime, and the WDT
+device node exists, Finit will start it and treat it as the elected
+watchdog service to delegate its reboot to.  This delegation is to
+ensure that the system is rebooted by a hardware watchdog timer -- on
+many embedded systems this is crucial to ensure all circuits on the
+board are properly reset for the next boot, in effect ensuring the
+system works the same after both a power-on and reboot event.
+
+The delegation is performed at the very last steps of system shutdown,
+if reboot has been selected and an elected watchdog is known, first a
+`SIGPWR` is sent to advise watchdogd of the pending reboot.  Then, when
+the necessary steps of preparing the system for shutdown (umount etc.)
+are completed, Finit sends `SIGTERM` to watchdogd and puts itself in a
+10 sec timeout loop waiting for the WDT to reset the board.  If a reset
+is not done before the timeout, Finit falls back to`reboot(RB_AUTOBOOT)`
+which tells the kernel to do the reboot.
+
+An external watchdog service can also be used.  The more advanced cousin
+[watchdogd](https://github.com/troglobit/watchdogd/) is the recommended
+option here.  It can register itself with Finit using the same IPC as
+`initctl`.  If the bundled watchdogd is running a hand-over takes place,
+so it's safe to have both services installed on a system.  For the
+hand-over to work it requires that the WDT driver supports the safe exit
+functionality where `"V"` is written to the device before closing the
+device descriptor.  If the kernel driver has been built without this,
+the only option is to remove `/libexec/finit/watchdogd` or build without
+it at configure time.