Create devdocs as .pdf and .html

[koppor]

Trying to generate pdf files out of devdocs - also supporting mermaid js

Idea from https://stackoverflow.com/a/78897319/873282

Refs just-the-docs/just-the-docs#1638

Fully rendered PDF available at https://jabref.github.io/jabref-koppor/jabref-devdocs.pdf

Mandatory checks

• I own the copyright of the code submitted and I license it under the MIT license
• [.] Change in CHANGELOG.md described in a way that is understandable for the average user (if change is visible to the user)
• [.] Tests created for changes (if applicable)
• [.] Manually tested changed features in running JabRef (always required)
• [.] Screenshots added in PR description (if change is visible to the user)
• [.] 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.