Add code of conduct, contributing guide and protect main

[PhilippTheServer]

Expected behaviour once solved

• The repository has a CODE_OF_CONDUCT.md that allows swearing about code but never at or about people.
• The repository has a CONTRIBUTING.md explaining the PR flow.
• .github/CODEOWNERS names @PhilippTheServer and @maltonoloco as owners of everything.
• main is protected by a ruleset: only org admins (@PhilippTheServer, @maltonoloco) may push directly; everyone else must open a PR that passes the CI checks and is approved by a code owner before it can be merged.
• CI fails if the community files are removed.

[PhilippTheServer]

Solved in #58: added CODE_OF_CONDUCT.md (swearing about code allowed, never at people), CONTRIBUTING.md, .github/CODEOWNERS (@PhilippTheServer @maltonoloco) and a CI step that fails if any of them goes missing. The main ruleset (PR + required CI + code-owner approval, org admins bypass) is applied via the API.