✨ feat(shortcodes): add remote_text shortcode

[welpo]

Summary

Adds the shortcode remote_text to allow embedding content from URLs or local files.

Related issue

Resolves #304.

Changes

• Introduced a new shortcode remote_text that supports both remote URLs and local file paths.
• The shortcode automatically differentiates between remote and local sources based on the prefix of the src attribute.

For example, to embed a remote Python script within a code block with syntax highlighting:

```python
{{/* remote_text(src="https://example.com/script.py") */}}
```

See the docs for more information.

Type of change

• Bug fix (fixes an issue without altering functionality)
• New feature (adds non-breaking functionality)
• Breaking change (alters existing functionality)
• UI/UX improvement (enhances user interface without altering functionality)
• Refactor (improves code quality without altering functionality)
• Documentation update
• Other (please describe below)

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Checklist

• I have verified the accessibility of my changes
• I have tested all possible scenarios for this change
• I have updated theme.toml with a sane default for the feature
• I have made corresponding changes to the documentation:
  • Updated config.toml comments
  • Updated theme.toml comments
  • Updated "Mastering tabi" post in English
  • (Optional) Updated "Mastering tabi" post in Spanish
  • (Optional) Updated "Mastering tabi" post in Catalan