Edit csharp variables entry

[braanj]

Description

Update the c# variables concept entry, by adding new discription, commonly used types definitions and examples.

Issue Solved

#5432

Type of Change

• Editing an existing entry (fixing a typo, bug, issues, etc)

Checklist

• [x] All writings are my own.
• [x] My entry follows the Codecademy Docs style guide.
• [x] My changes generate no new warnings.
• [x] I have performed a self-review of my own writing and code.
• [x] I have checked my entry and corrected any misspellings.
• [x] I have made corresponding changes to the documentation if needed.
• [x] I have confirmed my changes are not being pushed from my forked main branch.
• [x] I have confirmed that I'm pushing from a new branch named after the changes I'm making.
• [x] I have linked any issues that are relevant to this PR in the Issues Solved section.

[cigar-galaxy82]

Hi @braanj thank you for contributing to Codecademy docs, please make the suggested changes below.

I see that you have added data types instead of variables like:

Local variables Instance variables or Non – Static Variables Static Variables or Class Variables Constant Variables Readonly Variables

So please add the content related to these

[braanj]

Hey @cigar-galaxy82 the changes have been made!

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