Add xv6 article for COMP 3300

[benchittle]

What are you trying to accomplish?

Add an article for an overview of xv6 and related resources

How are you accomplishing it?

Adding the article

Is there anything reviewers should know?

No

Checks before you create your pull request

• [x] Did you read and follow article requirements?
• [x] Did you run prettier?
• [x] Is it safe to rollback this change if anything goes wrong?

[jere-mie]

Not sure if you're already on the contributors page @benchittle but if you aren't make sure to add yourself!