fix: help buttons targeting a not found page

[sad270]

The help button in Settings > PHP > Symfony > Method References is targeting https://espend.de/phpstorm/plugin/symfonyextension/method_parameter.html

The help button in Settings > PHP > Symfony > Type Provider is targeting https://espend.de/phpstorm/plugin/symfonyextension/signature_type.html

And both url are 404 Not found.

In this MR we replace url by those targeting PHPStorm documentation (the new documentation) https://www.jetbrains.com/help/phpstorm/symfony-support.html

As HELP_URL is only used on settings main page I remove the constant to replace it by a string. (Tell me if it's good for you)

I also change the buttonHelp label by Project Page / Documentation as it's not the Help page anymore. But the project Page and this project page contains documentation like. (I use the same wording we can found on plugin page here https://plugins.jetbrains.com/plugin/7219-symfony-support to be consistent)

[Haehnchen]

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