Replace well_known.rst with specs.md

well_known.rst was a remnant from the time when the Wayland color-management protocol draft contained its own enumerations of various video standard parameters. More recently all that was replaced by referring to H.273 CICP, making the list largely redundant with the H.273 document. Therefore well_known.rst is removed.

Still, well_known.rst contained a nice list of links to relevant specifications. Such a list could be useful, so it has been migrated to specs.md and expanded.