feat(i18n-es): add Spanish locale scaffolding

[Artificed]

Adds a Spanish locale, following the structure of the French one. This PR is the scaffold only, docs/es/index.md plus the es blocks in the VuePress config. Page translation follows by section.

Section links point at English until there's Spanish content behind them.

Verified on a clean build: /es renders with lang es-419, the switcher shows English, Français and Español, French output unchanged.

Note this puts Español in the switcher, which docs/pt does not do.

Part of #574

AI Assistance Disclosure
AI tools were used in producing part of this contribution.
Tools used: Claude Opus 5
Scope: initial draft of the Spanish page copy and the locale config entries
All AI-generated content has been reviewed, understood, and validated by the author.

[PaulMakinMojaloop]

I see no issues

[kayiganwa04]

Great work here. One thing I suggest is creating a master branch specifically for the Spanish translation. All changes should be merged into this branch until the translation is stable and fully completed.

However, we should not merge these changes directly into the master production branch until the Spanish translation is finalized.

[Artificed]

Thanks Eddy, done. Base is now i18n/es as you suggested, so this and the following Spanish PRs will merge there rather than master.