Correct Chinese language code, cross-link language versions

As also discussed in #1233

- corrected Chinese language code (`cn` => `zh`) to conform with [BCP 47](http://en.wikipedia.org/wiki/BCP_47); added custom redirect page at `/cn/index.html`
- added `{{page.section}}` metadata so as to be able to reference URI components (`site_url`, `page.lang`, `page.chapter`, `page.section`) individually
- refactored canonical meta tag in default layout to refer to the same page with trailing slash: `site_url/[page.lang/][page.chapter/][page.section/]`
- added alternate-hreflang meta tags in default layout to help search engines understand about different language versions
- added visible cross-language links in top navbar with `.navbar-right` styling for each page, using the URI components mentioned above to construct the different language links (essentially but not quite the same logic as for alternate-hreflang meta tags)
- included Bootstrap v3's `.navbar-right` styles in main.css, seeing as they seem to be missing from bootstrap.css at the moment and I don't want to mess with it. :D

It may be appropriate to edit README.md or add a CONTRIBUTING or TRANSLATING file to explain how to get started with new translations, and/or move translations to a more formal mechanism...