Refresh experiments_overview.markdown content for OpenVox

[miharp]

Summary

docs/_openvox_8x/experiments_overview.markdown contains several outdated references that need updating:

• Freenode IRC (line 46): irc.freenode.net is defunct — update to current community channels (Libera.chat, Slack, Discord?)
• Google Groups mailing lists (lines 45-46): puppet-users and puppet-dev — verify if still active or replace with current community links
• "Puppet employees" (line 18): Should reference Vox Pupuli / OpenVox maintainers
• "Puppet Enterprise customers" (line 22): No longer relevant; should be removed or updated
• "not officially supported by Puppet" (line 18): Should reference OpenVox/Vox Pupuli

The broken voxpupuli.org/docs/humans links on this page were fixed in #139.