libavif: Move documentation to docs variant or subport

[ryandesign (Ryan Carsten Schmidt)]

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 move libavif documentation to a docs variant or a libavif-docs subport so that depending on libavif is less costly.

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