[DOC]: Add docstring example to the contributing docs

[echedey-ls]

• [x] Partially addresses #2081
• [x] I am familiar with the contributing guidelines
• [x] Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`user`).
• [x] Pull request is nearly complete and ready for detailed review.
• [x] Maintainer: Appropriate GitHub Labels (including remote-data) and Milestone are assigned to the Pull Request and linked Issue.

Adds a docstring example to the contributing page. Now that I came back to the conversation in #2081 I see it was a discussion between @IoannisSifnaios and @AdamRJensen , so sorry for getting into it.

I still hesitate a bit on whether this is the preferred format to include this template here, maybe just linking to well documented functions is enough.

[cwhanse]

What do we think about these directives? https://www.sphinx-doc.org/en/master/usage/restructuredtext/directives.html#describing-changes-between-versions

I rather like them but don't want to introduce something that's a pain to manage. If we like them, it would be nice to show one in the example.

[kandersolar]

Thanks @echedey-ls and all reviewers :)