Consider moving "Getting Started" into its own section above User/Developer Guides

[mglaman]

One bit of feedback was that "Getting Started" was hard to find. You had to click "Developer Guide". This might not always been apparent to a user.

One suggestion was to move it into its own top level section. So our tree would be

Commerce 2.x

• Getting Started
• User Guide
• Developer Guide

That way it aligns with comments in #196 to guide users into the specific guides.

[calbasi]

I think it is a good idea :-)

[krcha]

Thank you, moved!