docs: add reference for "docker config" commands

This is mostly a copy of the equivalent `docker secret` commands,
which uses the same mechanisms behind the hood (hence, are 90% the
same).

We can make further refinements to these docs, but this gives us
a starting point.

Adding these documents, because there were some links pointing to
these pages in the docs, but there was no markdown file to link to
on GitHub.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

docs/reference/commandline/config.md

@@ -0,0 +1,35 @@
+---
+title: "config"
+description: "The config command description and usage"
+keywords: "config"
+---
+
+# config
+
+```markdown
+Usage:  docker config COMMAND
+
+Manage Docker configs
+
+Options:
+      --help   Print usage
+
+Commands:
+  create      Create a config from a file or STDIN
+  inspect     Display detailed information on one or more configs
+  ls          List configs
+  rm          Remove one or more configs
+
+Run 'docker config COMMAND --help' for more information on a command.
+```
+
+## Description
+
+Manage configs.
+
+## Related commands
+
+* [config create](config_create.md)
+* [config inspect](config_inspect.md)
+* [config list](config_ls.md)
+* [config rm](config_rm.md)