Add initial docs and example on testing with Playwright

[chuckcarpenter]

What kind of changes does this PR include?

• New or updated content

Description

This begins to add a testing section for information on how to add Playwright and a basic example test.

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[sarah11918]

Thanks for this, @chuckcarpenter and @tony-sull for jumping in with the assist! So excited to have some testing stuff here, and I think our community will really value and benefit from it! 🥳

I'll more closely review it tomorrow, but initially, if Playwright is to be one of several languages, then we're going to want to have an introductory paragraph before the Playwright header with some generic testing info, how testing works in Astro, WHAT you can test in Astro.. (testing your JS, testing what components render...), some popular testing tools that people use/have used in Astro. If either of you feel comfortable taking a stab that that, great! If not, I'll get to it tomorrow. You can look at some of the other pages to get a sense of the kind of "generic IN ASTRO..." kind of intros we usually have.

We should have something so that it's clear that this isn't the only way to test in Astro, and something that will include some popular words people might search for. So, even if we don't have any material on Jest, if someone searches for it (that maybe being the only testing tool they know?) then they'll end up here, with some info about how they CAN test, even if it's not what they originally expected.

[chuckcarpenter]

Committed suggestion, so back to you @tony-sull