Update the "Getting Started Guide" doc for WF 16

The "Getting Started Guide"[1] is release specific. This task is to update it for 16 before we release.

My expectation here is that this is mostly just changing a few places that say "15" to "16".

The "Requirements" section should not recommend using the latest update available; it should recommend the latest LTS available.

[1] http://docs.wildfly.org/15/Getting_Started_Guide.html