Improve contributors guide

[fhennig]

As a user of the SDP I might want to contribute features, and for that I want to know the guidelines around contribution and how to set up a dev environment.

Tasks

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1. make dedicated pages for "Contributing to an operator", "Contributing to documentation", "Contributing to product images"
2. Make sure the Steps to contribute are still up to date. This page should proably be changed significantly
3. Migrate nuclino meta-docs to the docs #578
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4. Give more helpful error messages in 'make run-dev' operator-templating#335
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5. Move 'building the operator' from Antora to Markdown secret-operator#375
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6. Move 'building the operator' from Antora to Markdown listener-operator#152
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7. Adjust README to be in line with other READMEs stackable-cockpit#210
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8. Improve the README image-tools#10
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9. Add a 'Project overview' page #556
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10. docs: update readme with contribution instructions operator-templating#354
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