docs: mention acls & sentinel are multi-region

* fix navigation issue for spread guide

* skeleton for preemption guide

* background info, challenge, and pre-reqs

* steps

* rewording of intro

* re-wording

* adding more detail to intro

* clarify use of preemption in intro

driver/docker: support unix destination mount path in windows

The reference architecture neglected to mention ACLs and Sentinel
policies are the exception to "no data is shared between regions."

Thanks to @notnoop for letting me know about this:
https://github.com/hashicorp/nomad/pull/6007#discussion_r308052454

Clarify encryption keys are shared across datacenters

use go1.11 in windows CI

#5973

document cgroup raw exec

Support for hot reloading log levels

This is an extraneous addition from #4612.

This also includes migration of some tests to async.

logmon: Workaround golang/go#29119

executor_universal_linux raw_exec cgroup failure is not fatal

comment formattinglogmon: fix comment formattinglogmon: fix comment
formatting

...and add a comment to switch back to the old error handling once we
switch to Go 1.12.

Fix links to namespace and quotas guides

There's a bug in go1.11 that causes some io operations on windows to
return incorrect errors for some cases when Stat-ing files. To avoid
upgrading to go1.12 in a point release, here we loosen up the cases
where we will attempt to create fifos, and add some logging of
underlying stat errors to help with debugging.

Clarify script checks requiring shell

Closes https://github.com/hashicorp/nomad/issues/5981

chore: Format hcl configurations

Update index.html.md