Feat(M&B): Custom invoicing

[lmilan]

Description

Fixes #4339

Note to reviewers

The broken link is to a page created in #4897

Preview Links

• https://deploy-preview-4867--kongdeveloper.netlify.app/metering-and-billing/custom-invoicing/

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).

[netlify]

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[Copilot]

Pull request overview

Adds a new Metering & Billing reference page documenting how to integrate external invoicing/payment providers via the Custom Invoicing app, intended to complement the existing Stripe integration docs.

Changes:

• Added a new /metering-and-billing/custom-invoicing/ reference page describing the Custom Invoicing integration flow and optional sync hooks.
• Documented UI steps for installing/configuring the Custom Invoicing app in Konnect Metering & Billing.
• Included a lifecycle/architecture overview (diagram + responsibilities split) for custom invoicing integrations.

[tomek-labuk]

Looks good!