docs: update slack links (#1990)
### Summary
A user in the [Community
Slack](https://unstructuredw-kbe4326.slack.com/archives/C043YA29U0J/p1698933003702919)
reported having difficulty signing up for Slack using the links from the
documentation. Updated the links to the use the invite link that worked
from him, which came from [this blog
post](https://medium.com/unstructured-io/setting-up-a-private-retrieval-augmented-generation-rag-system-with-local-vector-database-d42f34692ca7).
