Add link to ASV pvlib-benchmarker in documentation

[mikofski]

Problem
I can't find any reference or links to the ASV benchmarker in the documentation

Solution
I think a small section with a link to the ASV benchmarker and some discussion about why we benchmark pvlib, how to use read the results, and whatever

Alternatives
There are links in the pull request #1049

Additional context
The benchmarks are really cool, and it's impressive that we have them, so we should advertise and help people use them and solicit their feedback on how we can improve, what's useful, what's not, etc.

[kandersolar]

ICYMI, there technically is a trail of breadcrumbs that gets you to the benchmark graphs page, but it's a little roundabout:

• https://pvlib-python.readthedocs.io/en/stable/contributing.html#benchmarking
• https://github.com/pvlib/pvlib-python/blob/master/benchmarks/README.md

But it's not very discoverable and can certainly be made more useful.

[mikofski]

Agh! I searched different variations of benchmark(s) and benchmarker, but not "benchmarking" and evidently the sphinx/rtd search engine is very picky.

[wholmgren]

How about adding to the table of badges in the main readme? e.g.

<a href="https://pvlib-benchmarker.github.io/pvlib-benchmarks/">
<img src="https://img.shields.io/badge/benchmarks-asv-lightgrey" />
</a>

[Naman-Priyadarshi]

Working on this issue @mikofski

[wholmgren]

closed by #1427