docs: update README.md with current information

[amd-aliem]

Link to rendered doc, for easier review: https://github.com/amd-aliem/AMDSEV/blob/readme-rewrite/README.md

New:

• Add top-level heading and SEV Features table mapping SEV versions to EPYC platforms
• Add link to AMD SEV Enablement Guide (which uses new SEV X.X naming scheme for features)
• Add Branch Structure table documenting main vs tsm branches
• Add link to sev-certify repository for distro compatibility
• Add snphost ok recommendation for host readiness checks

Restructured:

• Add consolidated Upstream Version Reference matrix (replaces three separate tables)
• Consolidate HW and FW checks into single Prepare Host Section
• Make Branch Structure and Build subsections under Build and Install
• Add skip-note for users who don't need to build from source

Fixed:

• Fix typos: "Branhes" → "Branches", "succesful" → "successful", "satisify" → "satisfy"
• Fix double spaces and extra whitespace in FAQ section
• Fix extra space in [ How to increase SWIOTLB limit] link
• Standardize code fences from quadruple to triple backticks
• Escape *.deb / *.rpm glob characters in markdown
• Add missing punctuation for PCIe r6.1+ sentence