docs: backport gomega matchers from v4

[samuel-phan]

Backport documentation on gomega matchers from v4.

Checklist

• [x] documentation is changed or added

Description of change

Add more documentation about the available gomega matchers so the end-users don't need to look at the Golang file https://github.com/aelsabbahy/goss/blob/master/resource/gomega_test.go .

If the end-user doesn't know Golang, it can be harder to understand them.

fix #782

[samuel-phan]

Thanks, I'll see if I have time to dig further, but truth is: I don't know Golang, so my reading can only be superficial.

[aelsabbahy]

Closing this, as v0.4.0 is already in release-candidate: https://github.com/goss-org/goss/releases/tag/v0.4.0-rc.1

Thank you for the contribution though. Please feel free to open up new issues if the new documentation/goss version is not behaving as expected.