It turns page.size() and page.setSize() methods are slightly
confusing since there multiple different sizes (layout size,
content size, viewport size..)
This patch renames Page.{size,setSize} methods into
Page.{viewportSize,setViewportSize} methods to avoid confusion.
This patch adds docs/api.md file which contains API description
for the puppeteer API. This patch adds the API outline, which
doesn't not have explanations and samples.