feat(gateway): Upgrade and migration pages

[lena-larionova]

Description

Fixes #3367
Fixes #1330

This started off as the CE -> EE upgrade guide and snowballed into the gateway upgrade landing page, as I was trying to add more context. It felt more natural to present this as step 1 in a migration journey to Konnect.

• Adds a how-to guide for CE -> EE; we're purposely not adding a lot of detail here as every installation is different, and we can really only provide high-level guidance. For actual upgrade, they should use the upgrade guide (and work with PS).
• Adds a landing page for /gateway/upgrade/. This link takes over the old reference page link, which moves to /gateway/upgrade/reference/. We don't need a redirect since the original link still exists.

Preview Links

https://deploy-preview-6672--kongdeveloper.netlify.app/gateway/upgrade/
https://deploy-preview-6672--kongdeveloper.netlify.app/gateway/upgrade/reference/
https://deploy-preview-6672--kongdeveloper.netlify.app/gateway/upgrade/migrate-oss-to-ee/
https://deploy-preview-6672--kongdeveloper.netlify.app/gateway/install/#how-do-i-migrate-from-kong-gateway-open-source-oss-to-kong-gateway-enterprise

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[lena-larionova]

Redirect check is a false positive:

Missing redirects:
- Renamed app/gateway/upgrade/index.md, url: /gateway/upgrade
Error: Process completed with exit code 1.

This gets replaced by the landing page.

[cloudjumpercat]

LGTM!