New Function: array_keys()

[roberto-butti]

What

Add new method documentation file for a new PHP function in src/content/docs.
The instructions about defining the file's name are documented in the CONTRIBUTING.md file.

Description

Function name: example array_keys()
Function description: Returns an array containing all the keys of the
given array.
File name: example src/content/docs/02-array/08-arr-array-keys.md

Checklist

• create the file with the proper name
• create the documentation of the function
• write example in examples directory
• execute pint examples
• commit and open the PR.

Must read:

• https://github.com/roberto-butti/some-drops-of-php/blob/main/HACKTOBERFEST.md
• https://github.com/roberto-butti/some-drops-of-php/blob/main/CONTRIBUTING.md