Feat(M&B): Grants

[lmilan]

Description

Fixes #4336

Note to reviewers

I only ended up keeping a small part of the grants docs because it doesn't seem like there is any customization possible either through the UI or the API, which is what most of the content was about.

Preview Links

https://deploy-preview-4854--kongdeveloper.netlify.app/metering-and-billing/product-catalog/#grants

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).

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[Copilot]

Pull request overview

Updates the Metering & Billing Product Catalog reference to include concise documentation for “Grants” (per OM migration issue #4336) and fixes an internal anchor link.

Changes:

• Fix the Unit Cost warning callout link to point to the correct #rate-cards anchor.
• Add a new Grants subsection explaining usage allowances for metered entitlements.
• Remove a trailing blank line at the end of the document.