docs(tutorials): add STL loading to the Loading Models guide

[BenMalaga]

The Loading Models tutorial covers GLTF, FBX, and OBJ but not STL, which is one of the most common formats for 3D printing, CAD, and hardware work. This adds a short STL section in the same useLoader style as the others.

STL loads a little differently from the existing three, and the section calls that out:

• STLLoader returns a BufferGeometry rather than a scene or group, so you attach it to your own mesh with a material. The example shows that.
• STLLoader fills the normal attribute from the per-face normals in the file, so the geometry renders flat-shaded by default. The section notes that computeVertexNormals() is what you reach for when you want smooth shading or the file has missing/zero normals, rather than implying it is always required.

I left out a CodeSandbox demo to avoid referencing an asset that does not exist; happy to add one if you point me at a hosted STL model to match the other sections.

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[DennisSmolek]

If you can point to an open standard/test STL like the print boat or similar we could use that. CSB is being finicky, if you have an example you can pr it to: https://github.com/pmndrs/examples

There are big docs additions coming with v10 but I'm totally down to have a STL example