docs: Replace references to "mining" with "attestation"

[Vojtch159]

Updated mining to attestation, but maybe it doesn't even make sense to put it like this. This is for the maintainers to decide but maybe it'd make sense to remove "(which is called "attestation")" altogether and just add a link to more information about LMD-GHOST.

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[mehtavishwa30]

Will commit a small suggestion made to address @cameel's comment and merge ultimately. :)

Thanks for your contribution @Vojtch159!