docs: add overview of supported flux annotations *.toolkit.fluxcd.io

[srgvg]

Describe the bug

I was looking for an overview of supported Flux annotations, but noticed there is no such thing, only disparate, incidental mentions of specifcs, like kustomize.toolkit.fluxcd.io/purge/, kustomize.toolkit.fluxcd.io/ssa or kustomize.toolkit.fluxcd.io/substitute.

Steps to reproduce

n/a

Expected behavior

Having a specific section in the documentation describing all supported flux specific annotations.

Screenshots and recordings

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OS / Distro

n/a

Flux version

n/a

Flux check

n/a

Git provider

No response

Container Registry provider

No response

Additional context

No response

Code of Conduct

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