feat(ai-gateway): Add SageMaker provider + AWS authentication updates

[tomek-labuk]

Description

Fixes #6137 #6144 #6145:

• Add Amazon SageMaker provider reference page and landing page card
• Extract shared "Authentication with AWS" include used by Bedrock and SageMaker pages
• Convert both pages' config examples to entity_example (adds kongctl tab)
• Fix AWS auth field names to match the Konnect API spec (config.auth.*, no aws_ prefix)
• Fix SageMaker target region field name (region, not aws_region)

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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