`cmark` is a more robust library for constructing Markdown than
`lowdown`. It in particular has the functionality we need to compute the
doc we need to compute. It is also more portable.
For terminal output, there is a new A.I. slop library has the same
license as the C code from lowdown upon which it is based. This keeps
our options open for upstreaming, if we want to do that.
Also, the `builtins.fetchTree` docs are now assembled using the cmark
AST rather than janky string indenting.
