Update "Contributing to argopy" on Documentation

With a couple of new internal mechanisms to manage extensions, and going ahead of the 2025 training camps, I think we need to revisit and update the "Contributing to argopy" page on our documentation:

https://argopy.readthedocs.io/en/latest/contributing.html

This update should include:

- [ ] a new section about the [Argo accessor extension mechanism](https://argopy.readthedocs.io/en/latest/api.html#dataset-argo-xarray-accessor)
- [ ] an update to the [development environment section](https://argopy.readthedocs.io/en/latest/contributing.html#virtual-environment)

Start with the Contributing to argopy page at contributing.html, then read the linked Argo accessor extension documentation in api.html and the existing virtual environment section. Update the page with the extension mechanism and current development-environment guidance; done means both requested sections are complete and the documentation links and examples are accurate.

Written by the indexing model from the issue text.