Add getHelpIndex to Options panel for JWT

[preetkaran20]

Describe the bug
As the options panel for the JWT addon is complex, we should add the help index for the addon so that users can see the help index there only and they don't need to go to the readme.md in the repository to understand the options panel.

example of Help index:

Sample code changes
Help Index: https://github.com/SasanLabs/owasp-zap-fileupload-addon/tree/main/src/main/javahelp/org/sasanlabs/fileupload/resources/help

In https://github.com/SasanLabs/owasp-zap-jwt-addon/blob/master/src/main/java/org/zaproxy/zap/extension/jwt/ui/JWTOptionsPanel.java implement a method:

    @Override
    public String getHelpIndex() {
        return <name>;
    }

Sample PR: https://github.com/SasanLabs/owasp-zap-fileupload-addon/pull/6/files

Glimpse of UI where the help will be shown

Testing the changes
build the addon by running

1. ./gradlew spotlessApply
2. ./gradlew build
   Then go to the ZAP -> File -> Local addon file -> Navigate to project -> build -> bin -> jwt*.zap and done.