docs: improve extending matchers docs

[vorant94]

Description

This PR improves the docs for setting up custom matchers. With the current guide I wasn't able to properly configure TypeScript, so I needed to take a look at Testing LIbrary Jest DOM matchers how they handle it. Their approach worked for me, so I suggest updating the docs accordingly.

• change ambient declaration file to how it is configured inside testing lib since the current way didn't work for me
• rename MatcherResult to ExpectationResult as this is the current name of custom matcher return type according to source code (see MatchersObject > RawMatcherFn > ExpectationResult)

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