Update Volunteer Registration Link and Slack Link on zerobase.io/volunteers

[eriksogn]

The registration link needs to be this: https://airtable.com/shrLF5neOHKjsSkjD

The Slack link needs to be this: https://join.slack.com/t/necsi-edu/shared_invite/zt-dx9bg3ab-maC9RfhnHSGjMXkWGjS5Mw

Thank you!