Add incomplete HTML proposal draft.

Adapt the second draft into HTML format.  Add new function
"make_array."  Use the current draft version of Section 23.3.2
instead of my changes to ease adding/reverting my work.
Post third draft of the proposal.

Change which draft Standard to use as the base.  Use paragraph numbers.

Use actual tables in the changes for sections 20.11.7.4 and 23.2.3.

Change the wording in some section modifications or additions; copy all
sections of [array].  Use old typedefs in class template.  Rename
"element_type" to "dereference."  Add explicit converting constructor,
cross-type but same-shape.  Match stripping of "constexpr" from most
member functions.