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)