Add link to API documentation

[tjai89]

Add a link to API ActionController docs in the beginning of an article.

[rails-bot]

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[rafaelfranca]

Please add ci skip to the commit message

[utilum]

Speaking of links from guides to the API documentation, several guides have none. Would it be worth its own issue?

[tjai89]

Done

[tjai89]

I believe it makes sense to add links at the beginning of every article on its related api docs.