Combine transparency limitation notes into section in 3D rendering limitations

**Your Godot version:**

**Issue description:**
`The 3D rendering limitations` page already has a lengthy section on transparency *sorting*, but there are some other limitations of transparency that should be documented:
- All opaque objects render, then transparent objects render one at a time. This *is* noted already but since it's the root cause of many problems it should be clarified.
- Transparent objects do not show up on screen textures
- Some effects, like SSR or SDFGI, don't work with transparency
- Transparent object do not cast shadows (without a workaround)

These facts are currently documented in several places, across several pages. Sometimes they are duplicated, sometimes they are *not* duplicated and it causes problems.

These facts (and any other known limitations of transparency) should be added to the 3D rendering limitations page, as a new subsection of the existing [Transparency sorting](https://docs.godotengine.org/en/stable/tutorials/3d/3d_rendering_limitations.html#transparency-sorting) section. Existing notes on transparency limitations on other pages should stay as they are (or be slightly abridged), and also link to this new section.

**URL to the documentation page (if already existing):**
Existing limitations page: https://docs.godotengine.org/en/stable/tutorials/3d/3d_rendering_limitations.html#transparency-sorting
Existing notes on https://docs.godotengine.org/en/stable/tutorials/shaders/shader_reference/spatial_shader.html#light-built-ins,
https://docs.godotengine.org/en/stable/tutorials/shaders/screen-reading_shaders.html#behind-the-scenes,
https://docs.godotengine.org/en/stable/tutorials/3d/standard_material_3d.html#transparency (the note here actually lists pretty much all the problems. It should be the basis for the new section)