Adds XL machine spec and notes on large deployments (#4622)
Adds XL machine spec and notes on large deployments (#4622)

* Adds XL machine spec and notes on large deployments
* Clarifies machine sizes
* Fixes internal links within the document
* Moves datacenter size guidelines to "Single Datacenter" section

vamage/consul,vamage/consul,kingland/consul,kingland/consul,vamage/consul,kingland/consul,kingland/consul,vamage/consul,hashicorp/consul,kingland/consul,42wim/consul,youhong316/consul,vamage/consul,scalp42/consul,youhong316/consul,42wim/consul,youhong316/consul,scalp42/consul,youhong316/consul,scalp42/consul,42wim/consul,hashicorp/consul,youhong316/consul,42wim/consul,42wim/consul,scalp42/consul,hashicorp/consul,hashicorp/consul,scalp42/consul