[Merge after engine 2.24 release] Document rendering to cubemap faces and texture array layers

[mvaligursky]

Do not merge until engine 2.24 is released. It documents the RenderTarget layer option and embeds the render-targets/texture-array-impostors example, which are only available once engine 2.24 and its examples browser are released.

Documents rendering into a single cubemap face or texture array layer, and how render targets handle mipmaps, on the Render Targets page. Covers playcanvas/engine#9689.

Changes:

• New "Cubemap faces and texture array layers" section: the face and layer options, one render target per face / layer sharing a texture, depth buffer options (per render target, a depth array, or a shared 2D depth texture), and sampling a texture array in a custom shader (GLSL and WGSL)
• New "Sampling a texture while rendering into it" subsection: not possible on WebGL2; possible on WebGPU with a TextureView that excludes the rendered layer
• Embeds the new Texture Array Impostors example
• New "Mipmaps" section: automatic mipmap generation after each render pass, the mipLevel option, per-layer generation on WebGPU, and the WebGL2 workaround for rendering many layers (WebGL2: generate mipmaps only for the rendered cubemap face / array layer of a render target engine#9688)
• Intro, page description and "Cleaning up" mention cubemap faces and texture array layers
• Japanese translation updated to match