Squashed 'docs_source/' changes from 04619c0..f239f53

f239f53 Add ElastiCache to HA diagram.

git-subtree-dir: docs_source
git-subtree-split: f239f53a7942926ebfed9523080d9c27e4169f26
Merge commit '828a658fdebae636fd53380a177a6bc560c62d80'