New Function: substr and mb_substr

[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.

Highlight the differences between mb_substr and substr (with an example string with an emoji for example)

Description

Function name: example mb_substr()
Function description:
File name: example src/content/docs/05-string/18-str-substr.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