docs: clarify product positioning and entry paths

[abrichr]

Summary

• rewrite the documentation home page around product fit, the compiler output, execution, evidence, and next actions
• keep detailed deployment and qualification evidence on the detailed pages
• simplify the start, concepts, guides, commercial, and qualification entry copy
• align the docs source README with the public product truth

Validation

• documentation validation: passed
• mkdocs build --strict: passed
• pytest: 201 passed
• git diff --check: passed