Add schematic PDF(s) generation CI job

[apcountryman]

Add schematic PDF(s) generation CI job. The proposed design was outlined in #13.

• [ ] Add schematic PDF(s) generation CI script (ci/generate-schematic-pdfs)
  • [ ] Supported options:
    • [ ] --help: See other project scripts
    • [ ] --version: See other project scripts
  • [ ] function error(): See other project scripts
  • [ ] function abort(): See other project scripts
  • [ ] function display_help_text(): See other project scripts
  • [ ] function display_version(): See other project scripts
  • [ ] function main(): See other project scripts
  • [ ] function generate_schematic_pdfs():

    
    local schematics; mapfile -t schematics < <( git -C "$repository" ls-files '*.sch' ':!:*-sheet-*.sch' | xargs -r -d '\n' -I '{}' find "$repository/{}" ); readonly schematics

for schematic in "${schematics[@]}"; do if ! "$repository/utilities/KiAuto/src/eeschema_do" export -a "$schematic" "$repository/schematic-pdfs"; then abort fi done

- [ ] Add schematic PDF(s) generation (and archiving) CI job to `.github/workflows/ci.yml`:
```yaml
generate-schematic-pdfs:
    name: Generate schematic PDF(s)
    runs-on: ubuntu-20.04
    steps:
        - uses: actions/checkout@v2
          with:
              submodules: recursive
        - name: Generate schematic PDF(s)
          shell: bash
          run: ./ci/generate-schematic-pdfs
        - name: Archive generated schematic PDF(s)
          uses: actions/upload-artifact@v2
          with:
            name: schematic-pdfs
            path: schematic-pdfs
            retention-days: 7