Add Spanish translation for Introduction to GLSL tutorial

[xdroberto]

Adds the Spanish translation of the Introduction to GLSL tutorial, addressing the Spanish part of #1412 (the issue mentions intro-to-shaders.mdx, which was since renamed to intro-to-glsl.mdx). As discussed in that issue, I claimed the Spanish translation for this tutorial.

This creates src/content/tutorials/es/ — it would be the first tutorial available in Spanish.

Notes on the translation:

• 1:1 structural parity with the English original (862 lines): all code blocks, components, and markup are untouched, except code comments and describe() strings, which are translated following the convention of the existing Korean tutorial translation.
• GLSL keywords (attribute, varying, uniform, etc.) are kept literal when referring to code, and rendered as Spanish prose terms elsewhere.
• Only title, description, and featuredImageAlt are translated in the frontmatter; the {/* Note for contributors */} comment stays in English.
• Validated locally with the dev server: /es/tutorials/intro-to-glsl/ renders correctly and the tutorial appears listed on /es/tutorials/.

[ksen0]

Hi @marioguzzzman could you review please when you have time? Thanks in advance!