am 337b0873: docs: update backup guide and bmgr tool elaborate the guide to testing a backup agent and add links between the docs

Merge commit '337b0873f43eac5f23483bec94bd8729f79d47ec' into gingerbread

* commit '337b0873f43eac5f23483bec94bd8729f79d47ec':
  docs: update backup guide and bmgr tool