Add initial glossary

[koppor]

We have many terms floating around, which are implicitly explained. This is a structured approach to collect the terms.

I also went into SQOS with systems such as iQvoc, but that seemed to be overkill.

Steps to test

One could start Jekyll locally.

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.

[Siedlerchr]

Historically it was once called BibDatabase and thus you still find it in the code

[koppor]

Yeah, I added a hint at c052783 (#14248).

We (I ^^) should work on renaming this year to get it fixed. Too long postponed.