Redirect guides/vpn/overview.md to wireguard documentation

[yubiuser]

What does this PR aim to accomplish?:

Fixes #1393 by redirecting guides/vpn/overview.md to the wireguard documentation instead of the deprecated OpenVPN docs,

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[yubiuser]

@pi-hole/docs-maintainers what do you think about removing the OpenVPN documentation entirely? We added the deprecation notice two years ago....

[rdwebdesign]

what do you think about removing the OpenVPN documentation entirely?

We can do that in a follow up PR.