Add documentation for disabling GlyphIcons in Classic UI

[bluesky-adi]

What this PR does

Adds a new documentation section explaining how to disable GlyphIcons in Plone 6 Classic UI.

Why this is needed

Although GlyphIcons are no longer used by default in Plone 6, legacy themes or add-ons may still load them.
This documentation helps developers explicitly remove unused GlyphIcons resources and avoid accidental usage.

Scope

• Documentation-only change
• No code or behavior changes

Closes #694

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📚 Documentation preview 📚: https://plone6--2006.org.readthedocs.build/classic-ui/icons.html

[boring-cyborg]

Thanks for submitting your first pull request! You are awesome! 🤗

If you haven't done so already, read Plone's Code of Conduct and Contribute to documentation, as this will greatly help the review process.

Welcome to the Plone community! 🎉

[stevepiercy]

Thanks for the contribution! I've made some suggestions with explanations where needed. Please let me know if you have any further questions.

[stevepiercy]

See also this tip:

• glyphicons  Products.CMFPlone#1750 (comment)

But update its path accordingly:

• https://classic.demo.plone.org/@@theming-controlpanel#autotoc-item-autotoc-1

And use the {menuselection} directive using MyST (not reStructuredText) to have a nice rendering.

Note to self, I should add that to https://6.docs.plone.org/contributing/documentation/myst-reference.html, as well as guilabel for buttons, inputs, and other GUI elements.

[bluesky-adi]

Thanks so much for the detailed review and explanations !

I’ve applied all the suggested changes:

• Updated casing to GLYPHICONS® / GLYPHICONS
• Added links to GLYPHICONS and Bootstrap Icons
• Removed the redundant heading
• Polished wording as suggested
  Please let me know if you’d also like me to add GLYPHICONS to the Vale spelling dictionary, or if this PR should stay limited to documentation content only.

Thanks again

[stevepiercy]

@bluesky-adi I'm sorry to have wasted your time. Please see #694 (comment). We should have verified the original issue first.

[bluesky-adi]

No worries at all — thanks for the clarification and for the review.
Happy to verify behavior first before working on documentation next time.

Thanks again!