Docs: Add RegEx usage documentation for search and dynamic groups

[YerrabothulaNavyaSree09]

This PR adds missing documentation on how RegEx works in JabRef’s search and dynamic groups.

Based on maintainer suggestion in issue #9093:
“might be nice to put this into the RegEx documentation.”

Added:

• Examples of Java-style regex usage
• Tips for escaping special characters
• Dynamic group regex usage

[github-actions]

Hey @YerrabothulaNavyaSree09!

Thank you for contributing to JabRef! Your help is truly appreciated ❤️

We have automated checks in place, based on which you will soon get feedback if any of them are failing. In a while, maintainers will also review your contribution. Once that happens, you can go through their comments in the "Files changed" tab and act on them, or reply to the conversation if you have further inputs.

Please re-check our contribution guide in case of any other doubts related to our contribution workflow.

[jabref-machine]

You have removed the "Mandatory Checks" section from your pull request description. Please adhere to our pull request template.

[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).

[jabref-machine]

You modified Markdown (*.md) files and did not meet JabRef's rules for consistently formatted Markdown files. To ensure consistent styling, we have markdown-lint in place. Markdown lint's rules help to keep our Markdown files consistent within this repository and consistent with the Markdown files outside here.

You can check the detailed error output by navigating to your pull request, selecting the tab "Checks", section "Source Code Tests" (on the left), subsection "Markdown".

[ThiloteE]

issue #9093 has nothing to do with regex.

[ThiloteE]

Closing, because we already offer documentation about RegEx at this location in the documentation. If you want to continue working on this topic, then open another PR at the appropriate repository. Please avoid duplication in the documentation. You also have not addressed the automated feedback. Please avoid opening useless PRs, if there is no intent to keep working on your PR. Maintainers have to spend time on that, which they otherwise could use more meaningfully.