PSA: API documentation linking to source

[refack]

Original issue: nodejs/node#22393
First implementation: nodejs/node#22405
Example: https://nodejs.org/download/nightly/v11.0.0-nightly2018083060465700ed/docs/api/buffer.html#buffer_class_method_buffer_alloc_size_fill_encoding

/CC @nodejs/website-redesign

[refack]

H/T: @rubys

[fhemberger]

Thanks for the update!

Closing, because there's nothing left to do on the website.