Link Why CoCalc from a landing page and clean up on premises wording

[novoselt]

Description

• Add a (not very noticeable) link to "Why CoCalc?" doc page from the landing page
• Tweak on prem wording since we no longer have 3 options listed, but 2
• "Everyone's code" is a bit two generic, clarify "You and your collaborators"

Checklist:

• [ ] Testing instructions are provided, if not obvious
• [ ] Release instructions are provided, if not obvious