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
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documentation is changed or added -
commit message follows commit guidelines
Affected core subsystem(s)
doc