Rewrite initial Thema tutorial to focus on just basic schema authoring, then pivot to codegen right away

[sdboyer]

Thema's existing tutorial (parts 1, 2, and 3) is outdated and overly verbose.

All of part 2 and most of part 3 are now now pretty much done by codegen (thema lineage gen). thema lineage gen gobindings with --bindtype even goes a step beyond part 3, thanks to generics and the new vmux system.

I think a new pass at the tutorials should focus on getting a basic thema-based dev loop going: write a schema, update types and bindings in Go and TS, test with some data.

Subcommands within thema lineage init, thema lineage gen, thema data validate should be sufficient for all this. We can, for now, ignore multiple schemas and lenses.

Some of what's in the existing tutorials could probably be reused, but if we just started from scratch, i wouldn't be unhappy 😄