Documenting `Texture`, `TextureView` and `Sampler`.

[bzm3r]

Objective

Texture documentation is a little sparse, and it is difficult to say to what extent it can be improved, since it is ultimately a fairly light wrapper around wgpu's GpuTexture. Still, improvements could be made by adding relevant links to Texture related structures: for example, Image and TextureDescriptor.

Solution

Added a small blurb describing what Textures are used for, and noting its tight connection to Image. Also added a sentence noting that creating a texture usually requires setting up a TextureDescriptor. Similar light improvements were made to other related data structures.

[bzm3r]

@superdump Curious, as always, to hear your comments, if you have time!