feat: moved and updated prevOpenings.md

[LonelyTree]

• Moved previousOpportunities.mdto the outside of Onboarding Docs Folder.
• Updated previousOpportunities.md to include recent positions.

[manishprivet]

I think this link should be updated too alongside with it @LonelyTree

https://github.com/nodejs/mentorship/blob/main/README.md#previous-opportunities