Link to the slack at the bottom of the ToC (#6204)

Signed-off-by: Daniel King <dan@spiraldb.com>

Author: danking

docs/project/index.md

@@ -18,3 +18,4 @@ changelog/index
 - [GitHub](https://github.com/vortex-data/vortex)
 - [PyPI](https://pypi.org/project/vortex-data)
 - [Crates.io](https://crates.io/crates/vortex)
+- [Slack](https://vortex.dev/slack)