[Docs] Restore example of how to use font-awesome (refixes #184)

[josephtate]

Added documentation to addBadge, addSummary on how to use font-awesome's plugin api. This is a repeat of PR #184, which regressed during a documentation refactor, but instead of only being mentioned in the README, actually includes examples in the API docs.

Original commit: 55e3f5a
Refactor commit: fa18486

Testing done

These examples were pulled straight from my own Jenkinsfiles. No code was modified, only docs.

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[strangelookingnerd]

@josephtate Thanks!