"Factor" and "Texture" should not have separate headings

Pointed out by @lexaknyazev in https://github.com/KhronosGroup/glTF/pull/1825#discussion_r1846801385

The KHR_materials_transmission and KHR_materials_diffuse_transmission extensions both have section headings that separate "Factor" from "Texture". This distinction is purely mechanical in glTF, it's a choice between uniform and per-texel settings, and does not affect how a given PBR parameter is used in the material. For example:

https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Khronos/KHR_materials_transmission#transmissionfactor
https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Khronos/KHR_materials_transmission#transmissiontexture

https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Khronos/KHR_materials_diffuse_transmission#diffusetransmissionfactor
https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Khronos/KHR_materials_diffuse_transmission#diffusetransmissioncolorfactor
https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Khronos/KHR_materials_diffuse_transmission#diffusetransmissiontexture
https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Khronos/KHR_materials_diffuse_transmission#diffusetransmissioncolortexture

Ideally we should not use "Factor" or "Texture" in a section heading. The section heading should be just the name of the PBR parameter itself, such as "Transmission", "Diffuse Transmission", and "Diffuse Transmission Color".

Start with the documentation in extensions/2.0/Khronos/KHR_materials_transmission and extensions/2.0/Khronos/KHR_materials_diffuse_transmission, focusing on the linked Factor and Texture sections. Rename the headings to the corresponding PBR parameter names and verify that the resulting section links and references remain correct.

Written by the indexing model from the issue text.