📝 Add page on "What is property-based testing" and modify "Why property-based testing"

[josephjunker]

Description

As discussed in issue #6331, I think it would be helpful to tell new users what property-based testing is before trying to show them how to do it. I've added a new page at the front of the introduction which aims to do this. I've also made modifications to the "Why Property-Based Testing" page, largely to try to harmonize the two, but also to add some additional points which I feel were missed.

I'm happy to provide background on why I made each of the changes, if desired, or to make/revert any edits as requested.

Checklist

• The name of my PR follows gitmoji specification
• My PR references one of several related issues (if any)
  • New features or breaking changes must come with an associated Issue or Discussion
  • My PR does not add any new dependency without an associated Issue or Discussion
• My PR includes bumps details, please run pnpm run bump and flag the impacts properly
• My PR adds relevant tests and they would have failed without my PR (when applicable)

I'm not clear on whether I was supposed to run bump for a documentation change, so I left it out. Please let me know if that was incorrect!

Advanced

• Category: 📝 Add or update documentation
• Impacts: The "Introduction" section of the docs

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