HA overview added.

Some review fixes to HA installation guide added.
Merge pull request #932 from pasinskim/master

HA overview added.
Fixing layout of HA documentation.

Making documentation published to verify all the links.
Merge pull request #933 from pasinskim/master

Fixing layout of HA documentation.
Fixing page titles and links.
Merge pull request #934 from pasinskim/master

Fixing page titles and links.
Change: Fix typo in policy

Switching to paths provided by the stdlib I forgot to remove the
previously hard coded /bin/echo.
Merge pull request #935 from nickanderson/zendesk_1510

Change: Fix typo in policy
example for custom action script to ease development