Sync packit config by default when updating downstream

• We need to have the packit config in downstream and let is be up to date - let’s copy packit.yaml and spec file to downstream all the time - don’t require having it in synced_files
• update documentation
• We need some exclude-sync option in the config file to allow people to disable the sync of some files.

[TomasTomecek]

Some forward thinking: I can imagine that we would get more requests on the sync functionality, therefore I was thinking of introducing a new key sync which would accept a list of dicts, something like this:

sync:
- src: custom.spec
  dest: different.spec
- src: tests/*
  exclude: .*github.*

Therefore please before starting workin on this, let’s brainstorm the design.

[jpopelka]

related: #574