Make guide service centric

[lachmanfrantisek]

Acceptance criteria from the issue:

• [x] Onboarding guide can be used as the first page we can provide to the users.
  • No extra context is needed before reading this guide.
  • CLI is shown as a validation/debug tool.
• [x] Relation with Packit Service page is resolved.
  • Either by merging those two together or clear split with good linking.
• [x] The onboarding clearly shows how to set up Packit on GitHub/GitLab.
• [x] Relevant config options are linked (related to #467)
• [x] The jobs concept is well-described. Users can find and pick the relevant jobs. Also, it's easy to find more details and instructions.
• [x] The guide is easy to follow and not overwhelming (e.g. use screenshots and link the unimportant information).
• [x] Don't collide or merge with the release workflow guide: https://github.com/packit/packit.dev/issues/351

Fixes: #480

[softwarefactory-project-zuul[bot]]

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[softwarefactory-project-zuul[bot]]

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[softwarefactory-project-zuul[bot]]

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[lachmanfrantisek]

@FrNecas Thanks for such a detailed grammar&consistency check!

[softwarefactory-project-zuul[bot]]

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[softwarefactory-project-zuul[bot]]

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[morucci]

recheck

[softwarefactory-project-zuul[bot]]

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[softwarefactory-project-zuul[bot]]

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[lachmanfrantisek]

@jpopelka @lbarcziova @FrNecas @TomasTomecek Thanks for the reviews!

Anything else I should resolve/cover/add/update?

[softwarefactory-project-zuul[bot]]

Build succeeded.

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[lachmanfrantisek]

I am going to merge this but let me know (or create a new issue) in case of any additional suggestions/notes/...

Thanks for the reviews!

[softwarefactory-project-zuul[bot]]

Build succeeded (gate pipeline).

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