Ansible: add instructions for encrypting entire files

[pgerber]

Contributor checklist

• My contribution is ready to be merged as is
• I verified the modifications render properly
• I used spell checking
• I used the styling and structure aids available in Sphinx/ResT. (Links use `…`_, enumerations #., lists *, code ``…``/.. code::, warnings .. warning::, etc.)
• I reread the text keeping in mind that the text has to be comprehended fully by the target audience