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>