Documented Azure DNS deployment

[infbase]

Updated Helm chart, docs and examples for connecting to Azure Public DNS service.

Users should now be able to connect external-dns to an existing Azure Public DNS zone by setting the correct helm values, as demonstrated in the example deployment added to the docs in this PR.

Related to issue #642

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[ytsarev]

@infbase as discussed in community , let's provide ability to reference existing secret for more secure scenario