docs: add minirouter provider guide

[zhe-t]

Description

Adds a minirouter provider guide using Continue's generic openai provider with https://api.minirouter.sh/v1 and MINIROUTER_API_KEY

Also adds minirouter to the model-provider overview and sidebar navigation

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Checklist

• I've read the contributing guide
• The relevant docs, if any, have been updated or created
• The relevant tests, if any, have been updated or created

Screen recording or screenshot

Not applicable, this is a documentation-only change

Tests

• New page compiles with @mdx-js/mdx under Node 20.20.1
• docs/docs.json parses as valid JSON
• git diff --check passes
• Full Mintlify build is blocked before reading docs by the existing tar default-export incompatibility under Node 20.20.1 and Node 25

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[zhe-t]

I have read the CLA Document and I hereby sign the CLA.