docs: replace wiki links

See merge request !33

A format of "" would cause a blank edit field.  We do not want that.

Apparently gnome-text-editor offers precisely one format instead
of the usual dozen or so.  And it's an ad-hoc format.

xmllint goes into the deep end and doesn't finish in any reasonable
time, if at all.

Using a container that knows its length makes this introspectable.
At the same time also hold a ref for every style.  Most callers
do not actually need that -- for file format writers there is no
chance that the style disappears underneath the caller -- but it
doesn't hurt in that situation either.

Resizing in a way that would increase columns at the same time as
decreasing rows did not work right.

This is experimental.  There is no support available.  Please do not
file bug reports unless they have ready-to-commit patches.

This adds support for _Decimal64, a 16-digit base-10 floating-point
type.