Welcome tab

[Yubo-Cao]

Closes #12664

Video

Steps to test

1. Open JabRef
2. The improved Welcome Tab should be visible

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.

[jabref-machine]

Note that your PR will not be reviewed/accepted until you have gone through the mandatory checks in the description and marked each of them them exactly in the format of [x] (done), [ ] (not done yet) or [/] (not applicable).

[trag-bot]

@trag-bot didn't find any issues in the code! ✅✨

[jabref-machine]

JUnit tests of jablib are failing. You can see which checks are failing by locating the box "Some checks were not successful" on the pull request page. To see the test output, locate "Tests / Unit tests (pull_request)" and click on it.

You can then run these tests in IntelliJ to reproduce the failing tests locally. We offer a quick test running howto in the section Final build system checks in our setup guide.