docs: add Spanish (es) translation support for the website

**Task description**:
The Kmesh website currently supports documentation in English and Chinese. To improve accessibility and reach a wider global audience, this task proposes adding Spanish (`es`) translation support to the website using the existing Docusaurus i18n setup.

The initial focus will be on setting up the Spanish locale and translating a small set of core pages (such as the homepage and documentation overview), with the possibility to expand translations incrementally.

**Solution**:
- Add Spanish (`es`) as a supported locale in the Docusaurus configuration
- Create the initial i18n folder structure for Spanish
- Translate key entry pages (home, docs overview, basic navigation)
- Ensure the site builds and renders correctly with the new locale

This task does not require changes to core application logic and is well-suited for documentation-focused contributions.

**Who can join or take the task**:
This is a good first issue intended for first-time contributors who want to get familiar with:
- The Kmesh website repository
- Docusaurus i18n structure
- Open source documentation workflows

Basic knowledge of Markdown is sufficient. Prior experience with translations or Docusaurus is helpful but not required.

**How to join or take the task**:
Just reply on the issue with the message `/assign` in a separate line.
Then, the issue will be assigned to you.

**How to ask for help**:
If you need help or have questions, please feel free to ask on this issue.
The issue author or other members of the community will guide you through the contribution process.