doc,deps: document how to maintain ICU in Node.js

- update v8 guide to mention ICU
- move content from the tools/icu/README.md but leave a pointer

Fixes: https://github.com/nodejs/node/issues/26108


Co-Authored-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/30607



Reviewed-By: Ujjwal Sharma <usharma1998@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>