doc: massive refactor and simplification to migrate to mkdocs

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

doc/config/rescue.md

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+Rescue Mode
+===========
+
+Finit supports a rescue mode which is activated by the `rescue` option
+on the kernel command line.  See [cmdline docs](../cmdline.md) for how to
+activate it.
+
+This rescue mode can be disabled at configure time using:
+
+    configure --without-rescue
+
+The rescue mode comes in two flavors; *traditional* and *fallback*.
+
+> [!NOTE]
+> In this mode `initctl` will not work.  Use the `-f` flag to force
+> `reboot`, `shutdown`, or `poweroff`.
+
+Traditional
+-----------
+
+This is what most users expect.  A very early maintenance login prompt,
+served by the system `sulogin` program from util-linux, or BusyBox.  If
+that is not found in `$PATH`, the bundled `/libexec/finit/sulogin`
+program is used instead.  If a successful login is made, or if the user
+exits (Ctrl-D), the rescue mode is ended and the system boots up
+normally.
+
+> [!WARNING]
+> The bundled sulogin in Finit can at configure time be given another
+> user than the default (root).  If the sulogin user does not have a
+> password, or __the account is locked__, the user is presented with a
+> prompt: `"Press enter to enter maintenance mode."`, which will open
+> up a root shell *without prompting for password*!
+
+Fallback
+--------
+
+If no `sulogin` program is found, Finit tries to bring up as much of its
+own functionality as possible, yet limiting many aspects, meaning; no
+network, no `fsck` of file systems in `/etc/fstab`, no `/etc/rc.local`,
+no `runparts`, and most plugins are skipped (except those that provide
+functionality for the condition subsystem).
+
+Instead of reading `/etc/finit.conf` et al, system configuration is read
+from `/lib/finit/rescue.conf`, which can be freely modified by the
+system administrator.
+
+The bundled default `rescue.conf` contains nothing more than:
+
+    runlevel 1
+    tty [12345] rescue
+
+The `tty` has the `rescue` option set, which works similar to the board
+bring-up tty option `notty`.  The major difference being that `sulogin`
+is started to query for root/admin password.  If `sulogin` is not found,
+`rescue` behaves like `notty` and gives a plain root shell prompt.
+
+If Finit cannot find `/lib/finit/rescue.conf` it defaults to:
+
+    tty [12345] rescue
+
+There is no way to exit the *fallback* rescue mode.