Remove Slack docs (no longer a free account)

Signed-off-by: David Young <davidy@funkypenguin.co.nz>

mkdocs.yml

@@ -311,7 +311,6 @@ nav:
       - Contribute: community/contribute.md
       - Code of Conduct: community/code-of-conduct.md
       - Discord: community/discord.md
-      - Slack: community/slack.md
       - Matrix: community/matrix.md
       - Reddit: community/reddit.md
       - Mastodon: community/mastodon.md