Move docs to ko.build

[imjasonh]

This copies content from README.md into separate docs in docs/, and uses mkdocs-material to generate a static site that's served from GitHub Pages, in the gh-pages branch.

A preview of this site served from https://github.com/imjasonh/ko.build is available at https://ko.build -- when this merges and is published I'll update the domain to point to this repo's site and archive that one. I'll also update our README.md to link to ko.build instead of duplicating content.

This also moves generated CLI docs from doc/ to docs/reference/, so we don't have to keep two copies in sync.

[codecov-commenter]

Codecov Report

Merging #749 (5e43025) into main (9139f45) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #749   +/-   ##
=======================================
  Coverage   51.19%   51.19%           
=======================================
  Files          44       44           
  Lines        3313     3313           
=======================================
  Hits         1696     1696           
  Misses       1404     1404           
  Partials      213      213           

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[imjasonh]

Flipping this to Draft since we'll need a google GH org admin to set up the domain. This shouldn't block the 0.12 release.

[imjasonh]

I'd like to get this merged so we can start running ko.build out of this repo instead of out of my personal repo.

This change also sets us up for the ko.build importpath, which will let us change the module name in 0.13 (e.g., go install ko.build@latest, import "ko.build/pkg/build", etc.