✨ 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)
• 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