Create user-focused documentation on how to generate SBOMs with Fromager #1016
Description
Some suggestions from @LalatenduMohanty:
A how-to guide (docs/how-tos/generating-sbom.rst) — covering how to enable SBOM generation via settings.yaml, per-package purl overrides, where the output lands (.dist-info/sboms/), and a working example.
A section in docs/reference/files.md — documenting the fromager.spdx.json output file format and location. This file already describes other build artifacts like graph.json and constraints.txt.