docs: build a static docs site (MkDocs) from the existing docs/ tree #13

Observation

v1.4.1 reorganised the flat docs/ root into five thematic subdirectories (guides/, design/, budgets/, reference/, project/) plus a freshly-written docs/README.md index. Every markdown link resolves; structure is clean.

What we don't have: a rendered static site. GitHub renders Markdown one file at a time — navigation between docs/guides/OPERATIONS_GUIDE.md, docs/ops/cansat_standard.md, and docs/design/architecture.md requires clicking back and forward, and there's no search.

Why it matters

A handful of per-profile ops guides + 8 ADRs + 5 budget docs is the right size to start justifying a site. Competition judges and external reviewers expect a "Documentation" link that opens a searchable site, not a GitHub tree view.

Proposal — MkDocs + Material theme

Minimal setup:

mkdocs.yml            # nav derived from existing docs/ tree
docs/                 # already organised; no reorg needed
.github/workflows/    # publish to gh-pages on every push to master (see #5)

Nav structure (mirroring the index in docs/README.md):

• Home → docs/README.md
• Guides → docs/guides/*
• Ops (per profile) → docs/ops/*
• Design → docs/design/*
• Budgets → docs/budgets/*
• Reference → docs/reference/*
• ADR → docs/adr/*
• Project status → docs/project/*
• Hardware / reliability / quality / security (separate sections)

Definition of done

• mkdocs.yml at repo root.
• mkdocs serve renders every existing file without broken links.
• mkdocs build produces site/ cleanly.
• .github/workflows/docs.yml publishes to gh-pages branch on every master push (depends on CI: add GitHub Actions workflow (no .github/workflows/ currently exists) #5).
• README.md "Documentation" section adds a link to the live site (e.g. https://root3315.github.io/unisat/).
• Search works; ADR index links still resolve; docs/diagrams/*.svg embed correctly.

Depends on #5 (GitHub Actions). Does not change any existing markdown.