docs: update connection security section

[danieltprice]

• Add information about Neon Proxy to the security overview section
• Add Neon Proxy glossary entry Preview: https://neon-next-git-dprice-update-connection-secu-27bd96-neondatabase.vercel.app/docs/security/security-overview#secure-connections

Related to suggestion in Slack: https://neondb.slack.com/archives/C03ETHV2KD1/p1731849260151229

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Name	Status	Preview	Comments	Updated (UTC)
neon-next	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Nov 20, 2024 6:58pm