docs: correct CommonJS installation examples for SDK 6

[javorosas]

Summary

• Correct the CommonJS installation example in Spanish and English: it incorrectly showed an ESM import.
• Document const Facturapi = require('facturapi') for SDK 6 or later and normalize the TypeScript label.
• Searched the documentation for require('facturapi').default; no examples required removal. Existing API examples already use direct require.

Release coordination

Draft until Node SDK 6 is published. Direct CommonJS constructor support is introduced by FacturAPI/facturapi-node#121. Merge after that release is available on npm.

Validation

• Both modified installation guides compile with the repository's MDX compiler.
• git diff --check passes.
• Direct require is covered by the SDK's native compatibility tests.

[javorosas]

Superseded by #303. Both installation guides now use direct CommonJS require without a heading above the example; release coordination is recorded in the consolidated PR.