doc: massive refactor and simplification to migrate to mkdocs

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

doc/requirements.md

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+Requirements
+============
+
+Finit is capable of running on both desktop/server systems with udev and
+embedded systems that usually come with BusyBox mdev.  Some systems have
+systemd-udev or eudev today instead of the original udev, Finit probes
+for all of them at runtime and expects `/dev/` to be a writable file
+system using `devtmpfs`.  It is also possible to run on a statically set
+up `/dev` if needed.  It is however not a good idea to have both udev
+and mdev installed at the same time, this will lead to unpredictable
+results.
+
+At boot Finit calls either `mdev` or `udevd` to populate `/dev`, this is
+done slightly differently and on systems with udev you might want to add
+the following one-shot task early in your `/etc/finit.conf`:
+
+```conf
+run [S] udevadm settle --timeout=120 -- Waiting for udev
+```
+
+Finit has a built-in Getty for TTYs, but requires a working `/bin/login`
+or `/bin/sh`, if no TTYs are configured in `/etc/finit.conf`.
+
+For a fully operational system `/var`, `/run` and `/tmp` must be set up
+properly in `/etc/fstab` -- which is iterated over at boot.