mkdocs.yml: add proc_api.md to not_in_nav

Описание

Из лога MkDocs убраны неинформативные строки, относящиеся к разрабатываемой странице architecture/proc_api.md:

INFO    -  The following pages exist in the docs directory, but are not included in the "nav" configuration:
             - architecture/proc_api.md

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• Close #...
• Target branch: main
• Changes should be cherry-picked to 24.1: NO
• Staging: https://docs.binary.picodata.io/picodata/branch-.../
• See also !239 (merged)