Document of syntax in manifest.yml

We don't have yet a complete document of the grammar in manifest.yml file. The closest is [wskdeploy docs](https://github.com/apache/openwhisk-wskdeploy/tree/master/docs) but they are not exactly the same. We should have a complete doc for this in project Firefly, tentatively part of https://github.com/AdobeDocs/project-firefly/blob/master/guides/app-hooks.md.

Start by reviewing the syntax used in manifest.yml and comparing it with the existing wskdeploy docs. Check the tentative guides/app-hooks.md location in project Firefly, then document the complete manifest grammar there or in the agreed documentation location; done means the supported syntax is covered beyond the current partial reference.

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