Add a documentation page for implementing a new language toolchain

[lihaoyi]

The intent is not to be production ready, but as a pedagogical exercise to show how it can be done using Mill's various features.

Seems like either C or Typescript are plausible candidates as they both have a two-step compile -> link/optimize workflow that should exercise a good amount of Mill's functionality. The end result should be

1. a sample three-module build in that language
2. two upstream modules that can compile independently/in-parallel
3. each module should use some simple third-party deendencies
4. one downstream module links/optimizes it all together into a runnable executable