Add Origin.Path to transform template

[saswatamcode]

This PR adds Origin.Path to front/backmatter templates. This is useful in case of constructing links that require paths such as GitHub edit links in path matter and makes YAML DRY-er, for example,

    backMatter:
      template: |
        Found a typo, inconsistency or missing information in our docs?
        Help us to improve [Thanos](https://thanos.io) documentation by proposing a fix [on GitHub here](https://github.com/thanos-io/thanos/edit/main/{{ .Origin.Path }}) :heart:

Test case also added.

[bwplotka]

LGTM, thanks!