docs: update reporters guide

[sheremet-va]

Description

Fixes #8523

This guide needs some love, putting it in a draft until I can give it more time.

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[mike-lischke]

Looks good to me.

[AriPerkkio]

vitest/docs/advanced/reporters.md

Lines 9 to 19 in 0d00650

	## Extending Built-in Reporters
	In general, you don't need to create your reporter from scratch. `vitest` comes with several default reporting programs that you can extend.
	```ts
	import { DefaultReporter } from 'vitest/reporters'
	export default class MyDefaultReporter extends DefaultReporter {
	// do something
	}
	```

This is quite bad as it makes reporters part of public API. I think we've broken their public API many times in the past.