docs: link the @stablechain/sdk npm package in the SDK reference

[bastienrp]

Adds the @stablechain/sdk npm link to the SDK reference install section.

The SDK overview already links the package in two places; the reference's install section only showed the install command with no link. This adds it for consistency:

viem >= 2.0.0 is a peer dependency. The package is published as @stablechain/sdk.

English-only per the i18n golden rule; the bot will draft cn/ko. style:check 0 blocking, docs:build clean.

🤖 Generated with Claude Code

[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" (L83)
• ⚠ warning: marketing word "seamless" (L30)

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