align assets and build output directory naming conventions

[thescientist13]

Summary

Coming out of #1199 / #1215, now that Greenwood can automatically bundle more and more static assets (pretty soon we will plan on having an <img> optimization step too!) it started to make "claiming" the assets/ directory a little more "greedy" on Greenwood's part. Generally, most project's in the space, if having an "auto copying" directory tend to call it public/ instead.

However, our current output folder name is public, so we will probably want to change that as well, likely to something call build/ or dist/.

Details

1. Rename assets/ to public/ - https://github.com/ProjectEvergreen/greenwood/blob/master/packages/cli/src/plugins/copy/plugin-copy-assets.js#L8
2. Rename output directory to build - https://github.com/ProjectEvergreen/greenwood/blob/master/packages/cli/src/lifecycles/context.js#L12
3. Update documentation - https://www.greenwoodjs.io/docs/css-and-images/#assets
4. This might also require updating some build scripts and / or test cases that might have the hardcoded