Fix CI: bump devcontainer to 0.24, set up .venv for Sphinx build

[Copilot]

Two independent CI failures: the Build and run tests job broke when apt.llvm.org started rejecting its own SHA1-signed key (deprecated 2026-02-01), and Deploy Sphinx docs to GitHub Pages broke because docs/Makefile hardcodes ../.venv/bin/sphinx-build but no workflow ever created that venv.

Changes

• .devcontainer/Dockerfile — bump base image psedoc/cpp-devcontainer:0.22 → 0.24, which carries the updated LLVM apt keyring that satisfies the SHA1 rejection policy
• .github/workflows/gh-pages.yml — replace the bare pip install step with a proper .venv setup that mirrors devcontainer.json's postCreateCommand:

  - name: Set up .venv
    run: |
      python -m venv .venv
      .venv/bin/python -m pip install --upgrade pip
      .venv/bin/python -m pip install -r requirements.txt

  requirements.txt already lists sphinx and sphinx_rtd_theme, so the redundant pip install sphinx line is dropped.

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