Add files that aren't supposed to be built in not_in_nav

[XhmikosR]

Fixes the two build info messages.

INFO    -  The following pages exist in the docs directory, but are not included in the "nav" configuration:
  - abbreviations.md
  - guides/vpn/openvpn/deprecation_notice.md

The mkdocs config should probably be tweaked later to warn so that it's stricter.

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