:book: Update new provider docs to be more current

[cprivitere]

What this PR does / why we need it: This PR attempts to update the Developing providers section of the Cluster API Book. It updates examples to reflect the current scaffolding provided by kubebuilder, be in line with current cluster api conventions, and fix errors and inconsistencies.

/area documentation

[cprivitere]

@fabriziopandini I've implemented your suggestions, thank you for them.

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 0c197decee417a85dc997722d10e96a30c1c7c76

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/lifecycle stale

[sbueringer]

/assign

[sbueringer]

Thx!

/lgtm /approve

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