docs(dcgw): Add Transit Gateway CIDR editing capability note

[Copilot]

Description

Documents that users can edit CIDR blocks after initial Transit Gateway configuration, a capability not available for VPC peering connections.

Preview Links

https://deploy-preview-3692--kongdeveloper.netlify.app/dedicated-cloud-gateways/transit-gateways/

Checklist

• Tested how-to docs. If not, note why here.
• All pages contain metadata.
• Any new docs link to existing docs.
• All autogenerated instructions render correctly (API, decK, Konnect, Kong Manager).
• Style guide (capitalized gateway entities, placeholder URLs) implemented correctly.
• Every page has a description entry in frontmatter.
• Add new pages to the product documentation index (if applicable).

Changes

• Added note in Transit Gateway configuration section clarifying CIDR blocks are editable post-configuration
• Explicitly states this capability applies only to Transit Gateway, not VPC peering

The note appears immediately after the CIDR configuration step where users initially set these values, ensuring visibility when they need it.

Original prompt

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This section details on the original issue you should resolve

<issue_title>DCGW: Transit gateway CIDR editing</issue_title>
<issue_description>## Definition of done

• Update https://developer.konghq.com/dedicated-cloud-gateways/transit-gateways/ with information (probably a note) that users can edit CIDRs (Note: doesn't apply to VPC peering: "based on the thread below; this is only applicable to Tgw and not VPC peering; VPC peering you are defining the destination CIDR range and that would be for 1 VPC. There is a bug they are addressing specific to VPC peering, TLDR - no update to VPC peering. https://konghq.atlassian.net/browse/KCGW-2593"

Information

DRP: Josh W.
Slack channel: https://kongstrong.slack.com/archives/C09RGHLB2HF

Due date (optional)

?

Size

Small
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes DCGW: Transit gateway CIDR editing #3500

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