API Change: siteConfig headerLinks

[ericvicenti]

headerLinksInternal and headerLinksExternal should be replaced with one array in siteConfig: headerLinks.

Each headerLink is one of the following:

Link to Doc:

{
  doc: 'foo',
  label: 'Foo Doc',
}

Link to Page:

{
  page: 'foo/bar',
  label: 'Foo Page',
}

This is the page located at website/pages/[en/]foo/bar.js

Manual/External Link

{
  href: 'https://github.com/facebookexperimental/Docusaurus',
  label: 'GitHub',
  external: true,
}

Search Box

Specifies the location of the search box in the header

{search: true}

[deltice]

Changed in #53. Also added a way to specify locations of the languages dropdown using:

{ languages: true }

If translations is enabled and the languages dropdown location is not specified, it will default to be all the way to the right (left of the search bar if the search bar location is also not specified).