[Doc] Add Features chapter

[fzaninotto]

Problem

• the doc is getting larger and larger, and it's hard to get a bird's eye view of the features offered by react-admin
• newcomers sometimes miss the value of react-admin (and what makes is special)

Solution

• Add a new chapter, to be read just after the tutorial
• Update the documentation sidebar to make it visible
• Rename the documentation sections to make them beginner-friendly

[slax57]

👌

[davidhenley]

I would also say it's hard to keep up with the docs even as someone who's used RA extensively. You guys are always adding new features!! Love what you guys are doing!