feat(ai-gateway): add Skills API capability for OpenAI and Anthropic

[gperanich]

Description

Documents the new llm/v1/skills route type (AG-907, aigw-2.1.0) as an api-type AI Model capability, alongside batches and files. Updates the provider capability tables, the AI Model entity reference, and the shared Konnect-UI walkthrough copy.

Preview Links

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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[jbaross]

Provisional review and updates (note that no Konnect API is available for testing at time of writing):

• Removed anthropic example from model entiy page. This page includes some examples but is not intended to function as a how-to. Similar to the existing example that only covers the OpenAI provider we should only have one example for skills, a user can check the schema or future how-to's for additional cases.
• Re-framed the example to explain the concept of API type model entities in general since the existing doc didn't cover this