Reorganize and improve shader Uniforms section

[tetrapod00]

The shader uniforms section has gotten very long and disordered, with bits of it seemingly split up by unrelated notes. This PR reorganizes it, adds subheaders, and makes some other minor improvements. The text itself is mostly unchanged.

The new order is:

• Uniforms (which links to Setting uniforms from code)
  • Uniform hints
    • Info
    • Uniform hint table
    • Source color note
  • Uniform groups
  • Global uniforms
  • Per-instance uniforms
  • Setting uniforms from code
    • Info
    • GDScript to shader data type equivalences
  • Uniform limits (formerly a note)

The only thing I'm unsure about is whether Setting uniforms from code should be the very first subheader, or where it is now. It makes sense to mention how to set uniforms from the inspector or from code early, BUT that section has a very long table so I think near the end is better. As a compromise, I linked to the section from the first Uniforms section.

[tetrapod00]

Not a cherrypick because it depends on #10119 which is not a cherrypick