Update introduction.md

[veraulitinaYa]

Hi! I added some details about PHP arrays! When I was working on the Language List exercise, I didn't find any information on the existence of functions that are provided by PHP to work with arrays.
I thought adding some info on that would be helpful.

[mk-mxp]

Hello @veraulitinaYa and welcome to the PHP track!

Our concept exercises try to balance giving information and nudging towards exploration of the documentation. Putting all required functions into the introduction is, in my eyes, too directly giving away the information and preventing from "investing the effort to find the right solution".

There is a link to all array functions at the end of the concept itself. And we always have a hints section (in the online editor the "Get help" tab in the right sidebar may be hidden and you need to scroll to the tab).

With all that new information, would you still want to add the paragraph as it is? I would prefer to at least take out the direct mentioning of array_shift() and count() and instead add another link to the PHP array documentation. What do you think?

[veraulitinaYa]

Hello @mk-mxp! Thank you for welcoming me and giving helpful tips!

Your suggestion sounds great, I'll update the pull request and make another commit!