Bug 897034 - Improve discoverability of L20n, part 1

Split the main README in multiple files inside docs:
 - Add a second example with genitive to justify the more complex structure of
   brandName in Polish.
 - Invert JavaScript API and HTML bindings sections
 - Collapsed "Further reading" into JS API section, fix typo in JavaScript code
   (textConent -> textContent)

docs/build.md: add instructions to build l20n.js with node.js
docs/html.md: fix wrong line ending
docs/api: fix typo (textConent -> textContent)