docs: finish docs for launch

[lostmygithubaccount]

we need sufficient docs for someone landing on the project. in scope for now:

• landing page
• getting started tutorials
• concepts
• API reference

API reference can possibly be dropped out of scope

in the future, we would also want:

• how-to guides
• contributing guides
• (possibly) separate posts from Ibis; use Ibis posts for now

[lostmygithubaccount]

there are docs