misc/freebsd-doc-*: Enable HTML in all languages and Simplify ports

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