Spanish translation of the quickstart

[truthixify]

Extra labels: writing, i18n
Tier: M (2-4 days) | Type: writing

Context. Discord + application data show Spanish-speaking contributors are the largest non-EN cohort. Translate the flagship quickstart as a proof-of-concept for i18n on the doc site.

Scope.

• Full translation of guides/stellar/stellar-quickstart.mdx at guides/stellar/stellar-quickstart.es.mdx.
• Language switcher on the page.
• Doc the translation workflow so future guides are easier.

Acceptance.

• Native Spanish speaker reviews + signs off
• Language switcher works
• Contributors doc explains how to add more locales

Files. guides/stellar/stellar-quickstart.es.mdx (new), docs.json (nav).