doc: improve header styling for API docs

Sizes were mostly decided by using http://type-scale.com/
with the 1.250 "Major Third" scaling.

PR-URL: https://github.com/nodejs/node/pull/8811


Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Phillip Johnsen <johphi@gmail.com>