Feat(aigw): costs in migration guide

[jbaross]

Description

Adds description of new cost management parameters to concepts and migration guide docs.

Pending sanity check on what the converter does to old parameters may change language from 'add' to 'update'

Fixes #7060

Preview Links

https://deploy-preview-7065--kongdeveloper.netlify.app/ai-gateway/v2-migration-guide/

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]

I tested the migration guide with a config that included the V1 params

model:
          model_alias: null
          name: gpt-4o
          options:
            input_cost: 7
          provider: openai

and this was carried over to V2

targets:
      - allow_auth_override: false
        config:
          input_cost: 7
          type: openai
        name: gpt-4o
        provider: openai-1
        weight: 100

So users should only have to add any new parameters they want to use not manually transfer existing values

[cloudjumpercat]

Left some feedback!

• The original DoD mentioned updating the AI Model targets section, does that still need an update?
• I tested the conversion and the converter had no issues with the input and output cost v1 dimensions. I've recommended adjusting some of the wording so it's clear the v2 additional dimensions are optional, not required for a successful migration.

[cloudjumpercat]

LGTM!