STA-2472: Add Chainstack to RPC provider docs

[probablyangg]

Summary

• add Chainstack to the RPC provider overview
• document managed RPC nodes, HTTPS and WebSocket endpoints, and full/archive access
• add setup guidance and related next steps

Why

Chainstack now supports Stable, so developers need a direct path to discover and configure its RPC infrastructure.

Developer impact

Developers can find Chainstack alongside the existing Stable RPC providers and follow a concise setup path to deploy a node and copy its endpoint.

Validation

• npm run check under Node 22
• docs style check: 0 blocking findings
• i18n parity check: 0 missing pages
• TypeScript, structured-data, and docs build checks passed

Scope note

The StableHub ecosystem directory is populated through the Builder.io ecosystem-partners model, not this repository. Its Chainstack entry requires a separate CMS update.

[github-actions]

Styleguide check

✓ 132 en pages — 0 blocking, 11 warnings

docs/pages/en/explanation/autobahn.mdx

• ⚠ warning: marketing word "robust" (L20)
• ⚠ warning: marketing word "seamless" (L34)

docs/pages/en/explanation/consensus.mdx

• ⚠ warning: marketing word "robust" (L32)

docs/pages/en/explanation/ethereum-compatibility.mdx

• ⚠ warning: marketing word "seamless" (L10)

docs/pages/en/how-to/monitor-node.mdx

• ⚠ warning: marketing word "comprehensive" (L7)

docs/pages/en/how-to/troubleshoot-node.mdx

• ⚠ warning: marketing word "comprehensive" (L7)

docs/pages/en/reference/indexers.mdx

• ⚠ warning: marketing word "seamlessly" (L9)

docs/pages/en/reference/ramps.mdx

• ⚠ warning: marketing word "seamless" (L28)

docs/pages/en/reference/system-transactions-api.mdx

• ⚠ warning: marketing word "comprehensive" (L358)

docs/pages/en/reference/wallets.mdx

• ⚠ warning: marketing word "powerful" (L86)
• ⚠ warning: marketing word "seamless" (L33)

_{Enforced by npm run style:check — see STYLEGUIDE.md.}