Add Plugin Publishing Guidelines page and update sidebar links

[Xpirix]

Description

Closes #279

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• I have read the contributing guidelines and my pull request follows them.
• my commits tend to comply with Conventional Commits; so they are descriptive and explain the rationale for changes. Messages and description are self-explanatory to make the git log a readable story of the project.
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Reviewer's checklist

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[Xpirix]

Preview:

[timlinux]

Things to be explicit about:

• License allowed: GPL, AGPL. BSD, MIT? which ones
• Embedded binaries
• Cross platform requirements
• Advertising
• Language(s) - I think metadata should at least be in English
• Dependency management
• Our ability to ban plugins
• Dispute procedure
• Renaming plugins
• Code of conduct
• Politics / contentious topics

[timlinux]

Maybe this even needs a QEP ....