put start and step docs into their own topic, moved from playbook_intro

which is meant to be kept simple

docsite/rst/playbooks_special_topics.rst

@@ -17,3 +17,4 @@ and adopt these only if they seem relevant or useful to your environment.
    playbooks_prompts
    playbooks_tags
    playbooks_vault
+   playbooks_startnstep

docsite/rst/playbooks_startnstep.rst

+Start and Step
+======================
+.. versionadded:: 1.8
+
+.. contents:: Topics
+
+This shows a few special ways to run playbooks, very useful for testing and debugging.
+
+
+Start-at-task
+`````````````
+.. versionadded:: 1.2
+
+If you want to start executing your playbook at a particular task, you can do so
+with the ``--start-at`` option::
+
+    ansible-playbook playbook.yml --start-at="install packages"
+
+The above will start executing your playbook at a task named "install packages".
+
+
+Step
+````
+.. versionadded:: 1.1
+
+
+Playbooks can also be executed interactively with ``--step``::
+
+    ansible-playbook playbook.yml --step
+
+This will cause ansible to stop on each task, and ask if it should execute that task.
+Say you had a task called "configure ssh", the playbook run will stop and ask::
+
+    Perform task: configure ssh (y/n/c):
+
+Answering "y" will execute the task, answering "n" will skip the task, and answering "c"
+will continue executing all the remaining tasks without asking.
+