Removed GitBook, add howto contribute docs.

[mvandermade]

Also added the online submission step-by-step via github.com.

📑 Summary

Brief description about the content of your PR.

Removed the GitBook mention.

Added steps on how to update the documentation.
I included two ways on how to edit and submit the files.

Resolves #1093

📏 Design Decisions

Describe the way your implementation works or what design decisions you made if applicable.

I included the online editor functionality because I like to use it myself and requires no tooling on your own local computer.
However the Markdown editors on your local computer might be more powerful to use, so I included that option too.

📋 Tasks

Make sure you

• 📖 have read the contribution guidelines
• 💻 have added unit/e2e tests (if appropriate)
• 🔖 targeted develop branch

[knsv]

Thanks for your clarifications!