Update contributing guidelines: solar energy experience, GSoC, avenue to close PRs

[RDaxini]

Is your feature request related to a problem? Please describe.
Ongoing discussion between maintainers regarding vetting contributors and GSoC spam PRs, and avenues to close low-quality PRs, all three of which are linked.

Describe the solution you'd like
The contributing homepage is currently a table of contents.
We can keep that structure or change the default to the intro page. Either way, I propose updating the first paragraph of the intro page to include the following points:

1. pvlib-python is an advanced scientific modeling toolbox
2. we welcome new contributors
3. Reviewer capacity is limited
4. PR contributors should have a background in solar energy/PV
5. New contributors opening their first PR should clearly state that they have some background experience in PV
6. Maintainers reserve the right to close PRs that do not comply with the contributing guidelines
7. GSoC: TBD

Note on points 4–6:
all contributors are already required to read the contributing guidelines and tick the box to state they have done so in the PR template. Stating (or not) that they have PV experience serves as check of whether they actually did read the guidelines. At the maintainer's discretion, not reading the guidelines is sufficient grounds to close a PR because this is already a requirement for pvlib.

So, 4–6 serve three purposes: vetting contributing guidelines have actually been read, verifying the contributor has solar experience, clearly documenting a procedure/reason for maintainers to close PRs.

Describe alternatives you've considered
None really, whatever we do as a community to handle these issues is our decision that needs to be confirmed, but whatever it is that we do end up doing needs to be documented clearly and openly. The goal of this issue/PR is to document our policy and processes.

Additional context
I'm trying to keep the points being added to the docs simple and straightforward, and mostly in line with what we are already doing anyway. The hope is that this is therefore not a controversial suggestion and we can just get something clear in the docs on which we all agree.

Let's leave this issue open for a bit for feedback and we can reach a rough consensus then, if we are happy, I'll open a PR to finalize wording.

[cwhanse]

Rather than

4. PR contributors should have a background in solar energy/PV
5. New contributors opening their first PR should clearly state that they have some background experience in PV
6. Maintainers reserve the right to close PRs that do not comply with the contributing guidelines

What about

4. Every PR should address one or more open issues. The PR should be outlined in the issue discussion.
5. Maintainers reserve the right to close PRs that do not comply with the contributing guidelines
6. GSoC: For Google Summer of Code we invite applications only from those with significant PV modeling experience, and who are motivated to join the pvlib maintenance teams.

The GSoC deluge is the problem. We've had significant contributions from people with minimal PV experience, e.g., the entire structure that allows multiple Arrays in a PVSystem.

[wholmgren]

Agree that we've had significant contributions from people with minimal PV experience. 2014 Will had minimal experience - and 2026 Will has minimal experience compared to many here today too! Could we say something like:

A background in solar energy and/or experience in pvlib python is required to effectively address some issues due to subtleties that are hard to appreciate until struggling with an implementation. It is not always clear which issues fall into this bucket. Don't be offended if maintainers close your PR for this reason.

[RDaxini]

Good points, I agree! I am on board with removing that point and rephrasing as per both your suggestions.