fix: SDK sidebar categories link to wrong URLs

[MarketDataApp]

Problem

The sidebar categories for Stocks, Markets, and Options under the Go SDK link to /docs/sdk/stocks, /docs/sdk/markets, /docs/sdk/options instead of the correct paths /docs/sdk/go/stocks, /docs/sdk/go/markets, /docs/sdk/go/options.

These phantom URLs exist as pages (200 status) but don't correspond to any source file. The actual source files are at:

• sdk/go/stocks/index.mdx
• sdk/go/markets/index.mdx
• sdk/go/options/index.mdx

This likely affects PHP and Python SDK categories too (sdk/php/stocks/, sdk/py/stocks/, etc.) — they may all resolve to the same /docs/sdk/stocks URL instead of their language-specific paths.

How to reproduce

1. Visit https://www.marketdata.app/docs/sdk/go
2. Expand the Go SDK sidebar
3. Click "Stocks" — it navigates to /docs/sdk/stocks instead of /docs/sdk/go/stocks
4. Note the breadcrumb shows "Go SDK > Stocks" and the edit link points to sdk/go/stocks/index.mdx, confirming the source is under go/

Likely cause

The index.mdx files in sdk/go/stocks/, sdk/php/stocks/, sdk/py/stocks/ probably have a slug frontmatter field (e.g., slug: /stocks) that strips the language prefix from the URL. Each SDK's category pages need unique slugs or the slug should be removed to use the default filesystem-based paths.

Expected behavior

• Go SDK Stocks → /docs/sdk/go/stocks
• PHP SDK Stocks → /docs/sdk/php/stocks
• Python SDK Stocks → /docs/sdk/py/stocks
• Same for Markets and Options categories