✨ 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](https://welpo.github.io/tabi/blog/shortcodes/#remote-text) for more information.

### Type of change

<!-- Mark the relevant option with an `x` like so: `[x]` (no spaces) -->

- [ ] Bug fix (fixes an issue without altering functionality)
- [X] 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

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