This PR updates the checks documentation to mention support for checks
when using the Nomad service provider. There are limitations of NSD
compared to Consul, and those configuration options are now noted as
being Consul-only.
This PR creates a top-level 'check' page for job-specification docs.
The content for checks is about half the content of the service page, and
is about to increase in size when we add docs about Nomad service checks.
Seemed like a good idea to just split the checks section out into its own
thing (e.g. check_restart is already a topic).
Doing the move first lets us backport this change without adding Nomad service
check stuff yet.
Mostly just a lift-and-shift but with some tweaked examples to de-emphasize
the use of script checks.
