✨ feat: add previous/next article navigation

[welpo]

Summary

Adds the feature "Previous and Next Article Links" to display navigation links at the bottom of posts for moving to the previous or next article. This setting follows the settings hierarchy, meaning it can be set in the post, section or entire config.toml level.

Related issue

Resolves #244.

Accessibility

Uses semantic HTML and the contrast is the same as the rest of tabi.

Screenshots

Type of change

• [ ] 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
• [x] I have updated theme.toml with a sane default for the feature
• [x] I have made corresponding changes to the documentation:
  • [x] Updated config.toml comments
  • [x] 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

Acknowledgements

The logic for this feature was inspired by the Abridge theme.