Add reference to and clean up authentication-mechanisms.md

[windsonsea]

Performed actons below:

• remove redundant spaces
• provide a consistent look for bullets
• add some reference links to the bottom
• minor changes about typo and grammar

/kind cleanup

[k8s-ci-robot]

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[dipesh-rawat]

/lgtm

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 891c84f988a014fc8b495afc1a081a316a84ef21