Add headers to get-involved-overview in hacktoberfest branch

[madhens]

In the hacktoberfest branch, in pages/get-involved-overview.md, add the following level 2 headers:

Invitation Benefits of Contributing Why We Need You

[cdalangin]

Hi! Can I do this? And I just have to do the headers right?

[madhens]

Hello! We're participating with a small Hacktoberfest group, so if you'd like to join, please register at the following link! https://organize.mlh.io/events/4599-hacktoberfest-documentation-meetup-closing-event