docs(partials): Add Partials page to documentation

[nowNick]

Description

Added a Partials page to documentation to describe how to use new Kong Gateway Enterprise Edition feature.

Issue reference

KAG-5682

Testing instructions

Preview link

• Partials page
• Overview on Kong Enterprise
• Admin API specs -- TODO

Checklist

• Review label added
• Conditional version tags added, if applicable.

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[nowNick]

Hey @lena-larionova @Guaris
I'm trying to see whether my change to API Spec looks good but I cannot find it on Netlify preview. Do I need to generate it somehow? Or am I looking at a wrong place? 🤔
I'd like to see some info about Partials here: https://deploy-preview-8518--kongdocs.netlify.app/gateway/api/admin-ee/latest/#/partials

Could you help me out?