docs: add instance props section

[mickaelchanrion]

Hi there 👋 I noticed that the docs was not mentioning the properties attached to a Lenis instance so I thought it would be nice to add it. Could come handy to know at first glance how to get lenis.velocity or lenis.progress!

Some properties were voluntarily ignored as they seem quite "internal". (e.g.: animate, onScroll, virtualScroll, etc.)

[vercel[bot]]

@mickaelchanrion is attempting to deploy a commit to the Studio Freight Team on Vercel.

A member of the Team first needs to authorize it.

[clementroche]

Looks sharp, thank you !