✨ feat: support disabling of the font-subset in header

[Nizzlay]

Summary

I've created my own custom skin, that doens't use custom font, so I added a way to disable the font-subset.

Related issue

Changes

• Added setting for enable_subset, which is set to true by defaults
• Amended logic in header.html to check if the subset is enabled
• Updated mastering-tabi-settings blogpost (English version)

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 updated config.toml in tabi-start
• 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

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[welpo]

I like the idea. I'll try to improve the docs (translations + Custom Subset docs) before next Monday and merge.

Thank you!

[welpo]

I've:

• renamed the flag to enable_subset, for consistency (also updated your PR text, for future reference)
• updated/added documentation

Merging now.

Thanks!