feat: allow users to indicate files dir

[jorgepiloto]

After a discussion on #432, we thought about allowing users to specify the location of their documentation files. With these changes, projects with doc/ and examples/ folders can now run:

  doc-style:
    name: "Doc style ${{ matrix.folder }}"
    runs-on: ubuntu-latest
    strategy:
      matrix:
        folder: ["doc", "examples"]
      fail-fast: false
  steps:
    - uses: ansys/actions/doc-style@v6
      with:
        files: ${{ matrix.folder }}
        vale-config: ${{ matrix.folder }}/.vale.ini
        token: ${{ secrets.GITHUB_TOKEN }}

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[jorgepiloto]

Merging and patching later.