write contributing guide

We expect to give continuous maintenance to translated sites and to create new ones from Spanish to English.

The contributing file could give a response (with how-to-steps or external links) to the following questions:

- How to translate one episode? sample script to reuse as https://github.com/ropensci-review-tools/babeldown/issues/87 (also using [aeolus](https://github.com/ropenscilabs/aeolus) to improve the output?)
- How to review the first translation? I drafted a guideline in this [PR comment](https://github.com/epiverse-trace/epimodelac/pull/51#issuecomment-2978126625). I just noticed that for novice github reviewers, not able to pre-render it locally is an issue
- How to create a translation site? e.g., make a copy of this repo, changing the lines of the workflow.
- Does English to Spanish translation require a different process or GH-workflow compared to Spanish to English? with respect to the dovetail configuration `lang="es"` in the GH-workflow
- How to update the translated material? https://ropensci.org/blog/2024/01/16/deepl-update-babeldown/
- Should maintainers add to the monthly task the manual activation of GH workflows?

Having this registry could help others to use or evaluate this system too.

This contributing guide could be replicated in all translated tutorials. (Should this repo be a template to replicate on GitHub?)