Remove outdated parts of code-howtos/IntelliJ (and move good parts)

[koppor]

I found the "## Other notes on IntelliJ" part in docs/code-howtos/IntelliJ.md. They became obsolete in #10323. There, I forgot about those parts.

Mandatory checks

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[koppor]

I added https://stackoverflow.com/a/36570524/873282 as a howto.

Checked the rendering using Docker (explained at https://github.com/JabRef/jabref/blob/main/docs/README.md)

[github-actions]

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