Add doi-to-bibtex to examples and JabKit

[koppor]

Strongly demanded by the community. Was available inside JabRef; now we can offer as CLI.

Kind of follow-up to #14233.

We use CrossRef

Steps to test

Create and modify debug configuration for org.jabref.JabKit#main

Mandatory checks

• I own the copyright of the code submitted and I license it under the MIT license
• I manually tested my changes in running JabRef (always required)
• [/] I added JUnit tests for changes (if applicable)
• [/] I added screenshots in the PR description (if change is visible to the user)
• I described the change in CHANGELOG.md in a way that is understandable for the average user (if change is visible to the user)
• [/] I checked the user 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 updating file(s) in https://github.com/JabRef/user-documentation/tree/main/en.