Add usage examples to registry

[t0yv0]

Hello!

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Issue details

While there are a lot of examples in the repo, the registry does not show them prominently. eks.Cluster has 1 non-interesting usage example, and ManagedNodeGroup has none. There seems to be precedent for e.g. awsx.Vpc to publish examples to registry so perhaps this can be done here as well.

We might need to plan out which use cases we want to see made prominent.

Affected area/feature

[t0yv0]

Can answer #842 question possibly.