Onboarding tutorial based on tact-by-example

[verytactical]

While we do supply example contracts, they're still unordered and a bit confusing for newcomers.

What we probably want is some onboaring flow, similar to those implemented in web IDEs of other contract languages.

The thing that repeatedly came up in discussions was that IDE is also used by proficient users for prototyping, and we shouldn't annoy them with intrusive propositions to go through onboarding. Definitely no fullscreen overlays!

We should update https://tact-by-example.org and re-use it for this purpose.

P.S.: There might be a tour.tact-lang.org subdomain for this purpose or just ide.ton.org/tour page.