doc: massive refactor and simplification to migrate to mkdocs

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

doc/runparts.md

@@ -0,0 +1,49 @@
+Runparts & `/etc/rc.local`
+==========================
+
+At the end of the boot, when all bootstrap (`S`) tasks and services have
+started, but not networking, Finit calls its built-in [run-parts(8)][]
+command on any configured `runparts <DIR>` directory.  This happens just
+before changing to the configured runlevel (default 2).  (Networking is
+enabled just prior to changing from single user mode.)
+
+```shell
+runparts /etc/rc.d/
+```
+
+Right after the runlevel change when all services have started properly,
+`/etc/rc.local` is called.
+
+No configuration stanza in `/etc/finit.conf` is required for `rc.local`.
+If it exists and is an executable shell script Finit calls it at the very
+end of the boot, before calling the `HOOK_SYSTEM_UP`.  See more in the
+[Hook Scripts](plugins.md#hooks) section.
+
+
+### Limitations
+
+It is not possible to call Finit via signals or use `initctl` in any
+runparts or `/etc/rc.local` script.  This because Finit is single
+threaded and is calling these scripts in a blocking fashion at the end
+of runlevel S, at which point the event loop has not yet been started.
+
+The event loop is the whole thing which Finit is built around, except
+for runlevel S, which remains a slow procession through a lot of set up,
+with a few hooks and blocking call outs to external scripts.
+
+However, not all `initctl` commands are prohibited. Supported commands:
+
+ - `initctl cond`: only operate on files in `/run/finit/cond`
+ - `initctl enable/disable`: enabled run/task/service is activated on
+   the runlevel change from S to 2
+ - `initctl touch/show/create/delete/list`: `create`, provided the
+   non-interactive mode is used, again changes take effect in the
+   runlevel change directly after bootstrap
+ - `initctl -f reboot/poweroff/halt`: provided the `-f` flag is used to
+   force direct kernel commands
+
+> [!TIP]
+> you can set a `usr/` condition in `/etc/rc.local` and have a
+> service/task in runlevel 2 depend on it to execute.
+
+[run-parts(8)]:     https://manpages.debian.org/cgi-bin/man.cgi?query=run-parts