docs: Add Portkey as a supported provider

[narengogi]

What does this PR do?

Hey! Naren from https://portkey.ai here, love the project and I've been using it everyday (switched over from claude code lol),
So I thought I'd quickly add docs, just added portkey as a supported provider, did not add models (too much maintanence)

here's corresponding doc on portkey's website https://portkey.ai/docs/integrations/libraries/opencode#opencode

btw we have a well maintained source of models as well if you'd like to utilise that (https://github.com/Portkey-AI/models)

How did you verify your code works?

N/A (doc update only)

[github-actions]

Hey! Your PR title Add Portkey as a supported provider doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[github-actions]

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