doc: massive refactor and simplification to migrate to mkdocs

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

doc/config/index.md

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+This section provides an overview of Finit's configuration system. For
+detailed information on specific topics, see the individual sections in
+the navigation menu.
+
+
+Configuration File Syntax
+--------------------------
+
+The file format is line based, empty lines and comments, lines starting
+with `#`, are ignored.  A configuration directive starts with a keyword
+followed by a space and the rest of the line is treated as the value.
+
+As of Finit v4.4, configuration directives can be broken up in multiple
+lines using the continuation character `\`, and trailing comments are
+also allowed.  Example:
+
+```aconf
+# Escape \# chars if you want them literal in, e.g., descriptions
+service name:sysklogd [S123456789]   \
+    env:-/etc/default/sysklogd       \
+    syslogd -F $SYSLOGD_ARGS         \
+    -- System log daemon \# 1   # Comments allowed
+```
+
+The .conf files `/etc/finit.conf` and `/etc/finit.d/*` support many
+directives.  Some are restricted, e.g., only available at bootstrap,
+runlevel `S`.  Read on in [Files & Layout](files.md) for more on how
+to structure your .conf files.
+
+For details on restrictions, see [Limitations](limitations.md).