Consideration: detect `doc-kit.config.mjs` automatically

[bmuenzenmeyer]

Many tools that prescribe to config file flows will look for a config file automatically. This just bit me (my mistake) in a test app

auto-config-loader and cosmiconfig are examples.

We could expand our config loading to a library (perhaps not if we want to think about dependencies), or keep it tight and implement a happy path like the title suggests