[DOC]: Add docstring example to the contributing docs

[echedey-ls]

• Partially addresses GSoC feedback on the pvlib contribution process #2081
• I am familiar with the contributing guidelines
• 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`).
• Pull request is nearly complete and ready for detailed review.
• 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.

[RDaxini]

Nice job. I think this example will certainly help contributors in the future

[RDaxini]

Is it worth adding somewhere where such figures should be stored in the pvlib directory?

[echedey-ls]

I think that is intuitive enough if you see the repo layout and that most of the time us contributors copy&paste from other places. Feel free to weight in 👀

[IoannisSifnaios]

Personally this is exactly what I had in mind when we were discussing this! Nice work @echedey-ls! Some suggestions from my side.

[kandersolar]

This looks great to me. I bet an example like this will be a good reference not just for pvlib, but other packages too, and even people's personal code.