In the previous commit, the default value of the input is ${{ github.workspace }}
which means that most uses of this input would probably prefix their paths with
${{ github.workspace }}, especially since actions/checkout's 'path' input
must be under ${{ github.workspace }}. Therefore, it doesn't make much sense for
this to be an absolute file path.

Instead, it's more intuitive to make this relative to the repository.

This input is exposed in the CodeQL CLI as the flag --source-root, allowing
users of the CLI to set --source-root different from --working-dir. However,
in codeql-action, these two paths are conflated and it poses problems for
users with complicated build environments, in which a source root may be
a child of the working directory.

Most users should not notice this, as the default value is
${{ github.workspace }}, as it is implied now (`path.resolve()`).

Also, update the options and inputs documentation.

This input allows users to specify which packs to run. It works in
unison with the packs block of the config file and it is similar to
how `queries` works. They both use `+` in the same way.

Note that the `#TODO` in the pr check is still around, but the CLI
is available. I will remove the TODO in the next commit.