feat(docs-dev): add DocsDev image for the 415-docs review environment

[Sir-NoChill]

Summary

Adds a new top-level DocsDev/ directory with a Dockerfile that bundles
the toolchain cmput415/415-docs
needs for local docs builds and spec-review sessions:

• Sphinx + latexmk + texlive-latex-extra (matches deploySite.yml)
• lychee (matches linkcheck.yml)
• uv (Astral) for the pyproject.toml venv
• act — run the docs repo's GitHub Actions locally
• graphviz, gnupg, jq

Published as ghcr.io/cmput415/docs-dev:{date,latest} via a workflow that
mirrors the existing GazUtils / ANTLR / LLVM / DragonRunner build-and-push
pattern (.github/workflows/docs-dev.yml).

Why

Requested in review on cmput415/415-docs#110 — the .agents/ scaffold
there is being simplified to point at a maintained image instead of
provisioning the environment via a bash bootstrap script per session.
Agents (and humans) can docker run and get every tool the CI workflows
and the .agents/ scaffold expect on PATH, no host apt work required.

Test plan

• docker build ./DocsDev builds cleanly on the self-hosted runner.
• Inside the built image: uv --version, sphinx-build --version,
  lychee --version, act --version, latexmk --version, dot -V
  all report successfully.
• docker run --rm -v \$PWD:/workspace ghcr.io/cmput415/docs-dev:latest \\ bash -c 'uv sync && make -C gazprea html' succeeds in a checkout
  of 415-docs.
• The docs-dev.yml workflow tags and pushes both :latest and the
  date tag to ghcr.io on merge to main.

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