Docs: Add chapter about unit testing

[Obiwarn]

Many people around the web seem to struggle on setting up unit tests with Vite.

Any recommended frameworks, best practices?

[Shinigami92]

Just linking this for reference: #1955

[Niputi]

vitejs/awesome-vite#146

[patak-dev]

We discussed this in previous issues, and at this point, the best place for this information is awesome-vite as @Niputi suggested. I think we should close this one, as there are other issues open like Jest first-class integration that also imply documentation.