doc: add documentation on ICU

Adapted from the wiki article "Intl" by @srl295. Open to suggestions w.r.t. where this page belongs best, either here or on nodejs.org, but somewhere more permanent than a wiki.

Refs: https://github.com/nodejs/node/pull/13644#discussion_r121616327

Checklist

• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc