Jonathan devdocs merge conflicts

What does this MR do?

information gathered and added to devdocs: contributing.md, rebase.md, trubleshooting.md, summary.md

Please check if the information is correct and sufficient.

I didn't add screenshots as I don't know how to (both highlight the right part in phpstorm and add them to the devdocs). Maybe @peter.toennies might be able to do that?

How confident are you it won't break things if deployed?

Rather.

Links to related issues

Any relevant links (issues, documentation, slack discussions).

How to test

Steps a reviewer can take to verify that this MR does what it says it does e.g.

1. look at devdocs locally, specifically: contributing.md, rebase.md, trubleshooting.md

Screenshots (if applicable)

Any relevant screenshots if this is a design / frontend change

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md (description, merge request link, username(s))
• Once your MR has been merged, you are responsible to update the #foodsharing-beta Slack channel about what has been changed here. They will test your work in different browsers, roles or other settings