Pricing: remove "Basic plan" and add "Enterprise plan"

[humitos]

We are currently showing Basic, Advanced and Pro. We would like to change this to show Advanced, Pro and Enterprise.

We talked about keeping Basic below the fold in case there are people wanting to use Read the Docs at a lower price (school, universities, and simlar). However, most of the people should not even see this option 😄

• Enterprise plan should show support for SAML
• Keep the Basic plan in the comparison table
• Mention custom invoicing on Enterprise

[humitos]

We talked about doing #288 first and see how it behaves first.