Update unbound guide

[rrobgill]

Update file location and procedure for receive buffer size adjustment in Debian ≥13.
(#1293)

Provide note for older red-hat distros about location of unbound configuration files directory.
(#978, #470)

Include short note on checking Pi-hole's log to confirm use of unbound as upstream.
(#695)

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[yubiuser]

The direct attachment looked a bit strange.