doc doc.py

a page about 'page' and 'goal markdown'

gave src/java/examples/hello a README.md and a corresponding BUILD target

tweaked a couple of doc pages that "included" hello/main/BUILD for a code example: tweaked them so that they wouldn't mention the page target; would be strange in that context.

Bugs closed: 257

Reviewed at https://rbcommons.com/s/twitter/r/582/