✨ feat: add option to display/hide date in metadata

[welpo]

Summary

Adds show_date option to display/hide the date on pages. It follows the hierarchy.

Changes

Besides adding the option, this PR also adds logic to conditionally show the $SEPARATOR in the metadata. This avoids situations where the first element shown is the separator (e.g. hiding the date but showing reading time).

Related issue

Resolves #302.

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

• [ ] 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