Create implementation guide for the spec

The package spec contains descriptions of resources that can be installed in the Elastic Stack. Some resources need special management that is not always trivial ([example](https://github.com/elastic/package-spec/pull/307/files#r895515041)). It may be good to maintain in the spec a reference documentation for clients that implement the package spec, as Fleet.
This documentation may contain a list of general practices for the resources defined, as well as resource-specific considerations.
We may need to target different clients, e.g. installation use cases like in fleet vs. inspection use cases as in elastic-package or the registries.