doc: massive refactor and simplification to migrate to mkdocs

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

doc/config/sysv.md

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+SysV Init Compatibility
+=======================
+
+It is not possible to run unmodified SysV init systems with Finit.  This
+was never the intention and is not the strength of Finit.  However, it
+comes with a few SysV Init compatibility features to ease the transition
+from a serialized boot process.
+
+SysV Init Scripts
+-----------------
+
+**Syntax:** `sysv [LVLS] <COND> /path/to/init-script -- Optional description`
+
+> `<COND>` is described in the [Services](services.md) section.
+
+Similar to `task` is the `sysv` stanza, which can be used to call SysV
+style scripts.  The primary intention for this command is to be able to
+reuse much of existing setup and init scripts in Linux distributions.
+  
+When entering an allowed runlevel, Finit calls `init-script start`, when
+entering a disallowed runlevel, Finit calls `init-script stop`, and if
+the Finit .conf, where `sysv` stanza is declared, is modified, Finit
+calls `init-script restart` on `initctl reload`.  Similar to how
+`service` stanzas work.
+
+Forking services started with `sysv` scripts can be monitored by Finit
+by declaring the PID file to look for: `pid:!/path/to/pidfile.pid`.
+Notice the leading `!`, it signifies Finit should not try to create the
+file, but rather watch that file for the resulting forked-off PID.  This
+syntax also works for forking daemons that do not have a command line
+option to run it in the foreground, more on this below in `service`.
+
+> [!TIP]
+> See also [SysV Init Compatibility](#sysv-init-compatibility).
+
+`runparts DIRECTORY`
+--------------------
+
+For a directory with traditional start/stop scripts that should run, in
+order, at bootstrap, Finit provides the `runparts` directive.  It runs
+in runlevel S, at the very end of it (before calling `/etc/rc.local`)
+making it perfect for most scenarios.
+
+For syntax details, see the [Run-parts Scripts](runparts.md) section.
+Here is an example take from a Debian installation:
+
+    runparts /etc/rc2.d
+
+Files in these directories are usually named `SNNfoo` and `KNNfoo`,
+which Finit knows about and automatically appends the correct argument:
+
+    /bin/sh -c /etc/rc2.d/S01openbsd-inetd start
+
+or
+
+    /bin/sh -c /etc/rc0.d/K01openbsd-inetd stop
+
+Files that do not match this pattern are started similarly but without
+the extra command line argument.
+
+Start/Stop Scripts
+------------------
+
+For syntax details, see [SysV Init Scripts](#sysv-init-scripts), above.
+Here follows an example taken from a Debian installation:
+
+    sysv [2345] <pid/syslogd> /etc/init.d/openbsd-inetd -- OpenBSD inet daemon
+
+The init script header could be parsed to extract `Default-Start:` and
+other parameters for the `sysv` command to Finit.  There is currently no
+way to detail a generic syslogd dependency in Finit, so `Should-Start:`
+in the header must be mapped to the condition system in Finit using an
+absolute reference, here we depend on the sysklogd project's syslogd.
+
+`/etc/rc.local`
+---------------
+
+One often requested feature, early on, was a way to run a site specific
+script to set up, e.g., static routes or firewall rules.  A user can add
+a `task` or `run` command in the Finit configuration for this, but for
+compatibility reasons the more widely know `/etc/rc.local` is used if
+it exists, and is executable.  It is called very late in the boot process
+when the system has left runlevel S, stopped all old and started all new
+services in the target runlevel (default 2).
+
+> In Finit releases before v4.5 this script blocked Finit execution and
+> made it as good as impossible to call `initctl` during that time.
+
+`init q`
+--------
+
+When `/sbin/finit` is installed as `/sbin/init`, it is possible to use
+`init q` to reload the configuration.  This is the same as calling
+`initctl reload`.