Added a C cheatsheet by GeeksforGeeks

[Ajay-singh1]

What does this PR do?

Add resource(s) | Remove resource(s) | Add info | Improve repo Add resources

For resources

Cheatsheet

Description

Added a cheatsheet for C

Why is this valuable (or not)?

Yes

How do we know it's really free?

It is available on GeeksforGeeks website for free.

For book lists, is it a book? For course lists, is it a course? etc.

Cheatsheet

Checklist:

• [x] Read our contributing guidelines.
• [x] Search for duplicates.
• [x] Include author(s) and platform where appropriate.
• [x] Put lists in alphabetical order, correct spacing.
• [x] Add needed indications (PDF, access notes, under construction).
• [x] Used an informative name for this pull request.

Follow-up

• Check the status of GitHub Actions and resolve any reported warnings!