docs: refactor process docs

- split out run and enter docs into two different sections
- gather all process related docs into one new section
- add scheduled cron task docs

docs/processes/entering-containers.md

@@ -0,0 +1,29 @@
+# Entering containers
+
+> New as of 0.4.0
+
+```
+enter <app>  [<container-type> || --container-id <container-id>]  # Connect to a specific app container
+```
+
+## Usage
+
+The `enter` command can be used to enter a running container. The following variations of the command exist:
+
+```shell
+dokku enter node-js-app web
+dokku enter node-js-app web.1
+dokku enter node-js-app --container-id ID
+```
+
+Additionally, the `enter` command can be executed with no `<container-type>`. If only a single `<container-type>` is defined in the app's Procfile, executing `enter` will drop the terminal into the only running container. This behavior is not supported when specifying a custom command; as described below.
+
+By default, it runs a `/bin/bash`, but can also be used to run a custom command:
+
+```shell
+# just echo hi
+dokku enter node-js-app web echo hi
+
+# run a long-running command, as one might for a cron task
+dokku enter node-js-app web python script/background-worker.py
+```