Page tab panels

Nitdoc pages are tabulated. If a page has only one tab, it is presented as a single page. With more than one tab, the HTML rendering process adds tab headers and links.

Property definitions

nitc $ DocPage :: _tabs
	# Page tab panels
	#
	# Nitdoc pages are tabulated.
	# If a page has only one tab, it is presented as a single page.
	# With more than one tab, the HTML rendering process adds tab headers and
	# links.
	var tabs: Array[DocTab] = [main_tab] is lazy
src/doc/static/static_base.nit:97,2--103,45