fix duplicate page slug for i18n

[shibomb]

resolves #541

This is how we have handled it in the Japanese version ( https://p5js-ja.pages.dev/ ) .
We hope this will be helpful.

Changes

1. Remove slug property in .mdx files. (Slugs are automatically generated by Astro.)
2. Added process to convert locale prefix in .astro file, referring to other pages.

Steps to test

In your development environment.
e.g. Copy /src/content/pages/en to /src/content/pages/es
e.g. Copy /src/content/tutorials/en to /src/content/tutorials/es
and build.

Best regards