Update repository docs for GitHub-native workflow

[johnnygreco]

Objective

Update repository-facing documentation so contributors and maintainers see GitHub Actions and GitHub Pages as the docs CI/deployment workflow, with no stale GitLab CI guidance.

Context

• Tracker: GitHub goal delivery: convert repo from GitLab to GitHub #2
• Self-contained context: This repository is being converted from GitLab CI/Pages to GitHub Actions/Pages. The docs site is built from docs/ with Zensical, requirements-docs.txt, zensical.toml, and scripts/build-docs.sh. The first issue should establish the exact workflow file names and deployment path. Backward compatibility is not required.
• Supporting links: GitHub goal delivery: convert repo from GitLab to GitHub #2 and the PR from Add GitHub Actions CI and Pages deployment #3 once merged.

Scope

• In scope: update README.md and any other user-facing repository docs that mention GitLab CI or the docs deployment path; document the canonical local build/preview flow and the GitHub Pages deployment behavior.
• Non-goals: changing workflow logic, changing Zensical content model, or live Pages verification.
• Backward compatibility: do not preserve old GitLab instructions.
• Dependencies: Add GitHub Actions CI and Pages deployment #3 must merge first.
• Sequencing: second implementation issue; runs after workflow conversion so documentation can reference real workflow names.

Implementation Notes

Search for GitLab, .gitlab-ci.yml, GitHub Pages, GitHub Actions, and deployment references. Keep docs concise and aligned with existing README tone. The repository-level AGENTS.md instructions for static site development should remain valid.

Acceptance Criteria

• No stale documentation presents GitLab CI as the active docs build or deployment mechanism.
• README repository layout and documentation workflow describe GitHub Actions/GitHub Pages accurately.
• Local docs build and preview commands remain clear.
• Local validation commands pass after documentation edits.

Required Validation

• rg -n "gitlab|GitLab|\.gitlab-ci|gitlab\.io" README.md docs zensical.toml scripts .github returns no stale migration references, except intentional historical context if clearly marked.
• python3 scripts/render-dev-notes.py exits 0.
• python3 -m py_compile scripts/render-dev-notes.py exits 0.
• scripts/build-docs.sh exits 0.

PR Expectations

Include validation commands/results and identify all files with user-facing workflow guidance changes.

[johnnygreco]

Starting work on this issue from updated main after PR #6. I will update repository-facing documentation for the GitHub Actions/GitHub Pages workflow, run the tracker validation commands, and open a focused PR for review.

[johnnygreco]

Completed by PR #7, merged as f95d973. Validation: stale GitLab scan clean, renderer and py_compile passed, scripts/build-docs.sh passed, local site smoke returned 200, PR Docs workflow passed with deploy skipped on pull_request, and independent documentation/operations reviewers returned clean.