doc: mention smart pointers in Cpp style guide

Add rule for smart pointers, i.e., std::unique_ptr and std::shared_ptr,
to the Cpp style guide. Mostly copied from the Google style guide.

PR-URL: https://github.com/nodejs/node/pull/17055
Ref: https://github.com/nodejs/node/pull/16970
Ref: https://github.com/nodejs/node/pull/16974
Ref: https://github.com/nodejs/node/pull/17000
Ref: https://github.com/nodejs/node/pull/17012
Ref: https://github.com/nodejs/node/pull/17020
Ref: https://github.com/nodejs/node/pull/17030


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>