docs(site): GitHub Pages landing page polish

[seanseannery]

Summary

• Left-align terminal demo, 20s looping CSS animation, reorder install tabs (curl → Homebrew → npm)
• Rebrand to "Opsfile", fix tagline capitalisation, syntax-highlight Opsfile example, copy buttons on install blocks
• Style Step 2 as bash terminal blocks matching Step 1
• Moved site/ → docs/site/ and updated pages.yml path
• Add 2 feature cards: Team Standardisation, Secrets-Safe by Design
• Style flags table with cyan headers, left-aligned cells, orange code highlights
• Fix Opsfile example with accurate AWS CLI commands (aws logs tail, aws ec2 describe-instances)
• Add GitHub Discussions link to Links section
• Update copyright year to 2026, copyright holder to seanseannery
• Serve install.sh via GitHub Pages (seanseannery.github.io/opsfile/install.sh) for shorter curl URL; update README and site accordingly

Test plan

• Review site visually at the Pages preview URL after merge
• Verify copy buttons copy correct text to clipboard
• Verify terminal animation loops every ~20s
• Verify curl -fsSL https://seanseannery.github.io/opsfile/install.sh | bash resolves after Pages redeploy
• Confirm all install tabs (curl, Homebrew, npm) display correctly