Generate Markdown docs

[imjasonh]

This is largely copied from similar work in go-containerregistry

This required moving the Root command definition out of main() into a
place where it could be referenced from the gendoc tooling.

Rendered: https://github.com/imjasonh/ko/blob/docs/doc/ko.md

[imjasonh]

Two docgen diff issues:

1. --jobs defaults to GOMAXPROCS, which will be different for different users

-  -j, --jobs int                       The maximum number of concurrent builds (default 2)
+  -j, --jobs int                       The maximum number of concurrent builds (default 8)

2. --cache-dir (inherited from kubectl) will differ based on the user's homedir:

-      --cache-dir string               Default cache directory (default "/home/runner/.kube/cache")
+      --cache-dir string               Default cache directory (default "/Users/jasonhall/.kube/cache")

Sort of inclined to make both of these defined as zero values for the flag default, then interpret --jobs=0 as --jobs=GOMAXPROCS and --cache-dir as --cache-dir=$HOME/.kube/cache only at runtime.

[imjasonh]

We'll have more flexibility to change how --cache-dir is defaulted after (or as part of) #413

[jonjohnsonjr]

--jobs defaults to GOMAXPROCS, which will be different for different users

What if we did something stupid like replace The maximum number of concurrent builds (default 2) with The maximum number of concurrent builds (default GOMAXPROCS) via sed?

[imjasonh]

What if we did something stupid like replace The maximum number of concurrent builds (default 2) with The maximum number of concurrent builds (default GOMAXPROCS) via sed?

Defaulting -j at runtime seems more robust than relying on sed. See 903d3f4