Cardano Updates

1.  **PDF Jobs (`...-pdf`)**:
    * removed `uses: ./.github/workflows/build_artifact.yml`;
    * added build steps (Nix install, `fls-shake`, upload) directly into job;
    * Most importantly, there are now *two* checkout steps; the first checks out current branch (`master-markdown`); the second uses `sparse-checkout` to grab *only `src` directory* from `legacy-latex` branch and places it into workspace, overwriting `src` from `master-markdown`;
    * When `fls-shake` runs, it uses build environment from `master-markdown` (our `flake.nix`, etc.) but operates on legacy LaTeX source.

2.  **MkDocs Job (`mkdocs-site`)**:
    * new job runs only on pushes/PRs to `master-markdown`;
    * uses `_build` artifact from initial Agda compilation;
    * calls `nix develop .#markdown --command "cd _build/mkdocs/src && mkdocs build --site-dir ../../../site"`;
    * resulting `site` directory is uploaded as new artifact named `mkdocs-site`.

3.  **Artifacts Job (`upload-artifacts`)**:
    * job now depends on new `mkdocs-site` job;
    * downloads the `mkdocs-site` directory along with all other artifacts and commits them to `*-artifacts` branch, integrating new website into existing artifact deployment process.

attempting to preserve history in renamed Introduction.lagda.md file

I can't find a way to launch the server from the PROJECT_ROOT and also
have the global links file working, even if I specify the `mkdocs.yml`
config file location explicitly with the `--config-file` flag.  So (for
now at least) we must CWD to the `_build/mkdocs/src/` directory before
running `mkdocs serve` to serve/test the site locally.