docs(providers): add Rapid-MLX local provider section

[raullenchai]

Issue for this PR

N/A — new provider documentation addition.

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds a Rapid-MLX section to the Providers docs page
(packages/web/src/content/docs/providers.mdx), alphabetized between
OpenRouter and SAP AI Core.

Rapid-MLX is an
OpenAI-compatible inference server for Apple Silicon built on mlx-lm.
It works with the existing @ai-sdk/openai-compatible provider — the
section shows a complete opencode.json config snippet following the
same pattern as the existing LM Studio section.

A companion PR is open at anomalyco/models.dev#1361 to add the
rapid-mlx entry to the canonical provider TOML database.

How did you verify your code works?

• Verified the opencode.json config snippet works locally against a
  running Rapid-MLX server (model list + chat completions).
• Confirmed the section renders correctly in MDX by checking the
  existing LM Studio section as a structural template.
• Single-file docs change, no code modifications.

Screenshots / recordings

N/A — docs-only change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR