Add i18n / internationalization support

[AmitoVrito]

Description

Set up internationalization — start with English + Chinese.

What to do

• Configure Docusaurus i18n in docusaurus.config.ts
• Set up translation workflow (Crowdin or manual)
• Add language switcher to navbar

[AmitoVrito]

Thanks for your interest! I will assign it to you. Before you start, could you give the repo a ⭐ on the main repo (https://github.com/SynapseKit/SynapseKit)? It really helps with visibility for a new project. Looking forward to your PR!