Add indicator of target CocoaPods version for guides.

[jcampbell05]

When viewing a guide for Cocoapods (i.e Podfile or Podspec Syntax Guides) we should add an indicator for the target version the guide is written for:

These guides are up-to-date for CocoaPods $version

That way I know if the guide is for the version of CocoaPods I am using.

[orta]

Some other ideas:

Inline title:

Expanded nav selection:

[jcampbell05]

@orta ooh, I like those.

I think the first works great for a v2 , the nav selection version would be great if we added the ability to select versions of the guide as I think a user may think the version is something they can navigate via (since its on the nav).