What is the change?

Created two files stack_intro.md and stack_example1.md .
Created a folder Linked List added linkedlist.md to it.

Related issue?

closes: #82

Checklist:

Before you create this PR, confirm all the requirements listed below by checking the checkboxes [x]:

• [ y] I have followed the Code of Conduct.
• [ y] I have checked there aren't other open Pull Requests for the same update/change.
• [y ] I have you tested the code before submission.
• [y ] I have commented my code, particularly in hard-to-understand areas.
• [y ] My changes generates no new warnings.
• [y ] I'm Hacktoberfest'21 contributor.

Screenshots (if any)