provide a clear, step-by-step guide for doing releases with Packit

[TomasTomecek]

I just had a nice chat with @VladimirSlavik wrt upstream project releases and how those fit into the packit's upstream->downstream workflow. I just briefly went through our existing docs and it's true we don't have a clear and comprehensive guide for this. Let's create one!

I propose to create a new document solely for propose-downstream and update these 3 docs:

• https://packit.dev/docs/packit-service/#how-to-propose-a-new-downstream-update
• https://packit.dev/docs/guide/#packit-service-handles-fedora-rawhide-updates-for-you
• https://packit.dev/docs/cli/propose-downstream/

TODO:

• Convert https://packit.dev/posts/downstream-automation/ into a guide
• Describe config options that are utilized in the propose_downstream workflow
• Mention that Version in spec needs to correspond to the new release (related propose-upstream does not adjust the spec's Version: packit-service#1515)
• Have a dedicated section for archive generation and hosting, suggest github actions, link how we do it
• clearly describe distinction b/w --local-content and the default behavior
  • mention that HEAD is being used nevertheless

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[lachmanfrantisek]

I've updated that to match the whole downstream workflow.