Several improvements with this change:
Overall cleanup.
Simplify plists.
Make the HTML option default for all languages.
Non-English ports must build the HTML English version together, and the
shared files are stored in the freebsd-doc-en port.
Fix some image's src addresses on freebsd-doc-en.
Point MASTER_SITES to LOCAL/doceng; it's a shared folder with the team.
It's necessary to choose at least one option.
Reviewed by: doceng (blackend)
Differential Revision: https://reviews.freebsd.org/D39632
dad58c1094