feat: Add test tools docs

[hampuslavin]

Descrption / reasoning

The idea is to divide into different pages that has different focus areas. Currently I have the following in mind:

• Get started: Easily digested get started info, minimum for getting the simplest test to run
• The basics: Additional documentation you will absolutely need to know within "the first hour" of writing tests
• Advanced exmples: More complex test setup examples. Can still be common cases but not something you necessarily need to write a first test suite.
• Best practises: Inspired by Cypress' (e2e web test framework) best practices page, as inevitably there will always be cases where a user can do a thing in multiple ways. I've found the Cypress page to be tremendously helpful over the years. It also helps to educate and onboard team members who are new to a framework to quickly get into the "philosophy" of the tool.

❓ TODO / Question for reviewer:

• Consider what more best practises to add. Maybe some general testing principles that applied outside of Serverpod as well?

[SandPod]

Awesome work!

I like the structure presented here, have a couple of comments for different sections.
Ping me if anything is unclear!

[SandPod]

Some more minor NITs, do you want to convert this to a PR instead of it being a draft?

[SandPod]

Couple of minor last comments!

Then I think this is good enough to release :)

[SandPod]

Great work!

I think this looks good to me! :D Let's test it out and see if people understand it :)