libavif: Don't enable +docs variant by default, or move documentation to subport

[ryandesign (Ryan Carsten Schmidt)]

In its +docs variant, which is on by default, libavif depends on pandoc to produce its documentation. pandoc is a heavy dependency because it depends on ghc and ultimately over 500 other ports. gd2 is included in its recursive dependencies which means that gd2 cannot enable its libavif support.

Please don't enable the +docs variant by default. Alternately, move the documentation to a libavif-docs subport.

This was previously mentioned in #69332, #69566, and #70546.

[ryandesign (Ryan Carsten Schmidt)]

Reworded because libavif already has a +docs variant.