Getting started guide fixes & Explain X-SendFile a little better, with links

Explain how form_for :article is able to pull in the properties of @article
Make it clear that article_id is generated due to the association set up

Add link to the rails function that uses X-Sendfile.
Add links to apache and nginx docs for the header
Merge pull request #13937 from ktaragorn/guides_fixes

Guides fixes