doc: improve http2 documentation

Provide section headings for server-side and client side examples.
Add error handling and TLS to server-side example, following example
of `https`. Add error handling, TLS, more efficient Buffer usage,
and header printing to client example.

PR-URL: https://github.com/nodejs/node/pull/16366
Fixes: https://github.com/nodejs/node/issues/16345


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>