chore(docs-site): align homepage types and tsconfig with docusaurus 3.10

[lukecotter]

Summary

Updates the docs-site TypeScript setup to match the current Docusaurus 3.10 template: switches the homepage feature component off the deprecated global JSX.Element and tightens tsconfig.json for editor type-checking.

Changes

• Import ReactNode from react and use it in place of JSX.Element for FeatureItem.description and the HomepageFeatures return type (the global JSX namespace is gone in React 19 / TS 6).
• Enable strict: true and ignoreDeprecations: "6.0" in lana-docs/tsconfig.json so IDE type-checking matches what the upstream template ships.
• Exclude .docusaurus and build from the IDE TS project to avoid noise from generated output.
• Refresh the leading comment to reflect that the config is IDE-only (not used by docusaurus start/build).

Test plan

• pnpm --filter lana-docs typecheck
• pnpm --filter lana-docs build
• manual smoke of the homepage in pnpm --filter lana-docs start