feat: data-driven custom admonitions

[banteg]

Approach

This PR makes custom admonitions data‑driven and open‑ended in the UI while staying compatible with MiniJinja and existing theme semantics:

1) Consume arbitrary admonition types from config

• Read config.extra.admonitions for icon/color definitions.
  • Supports both shorthand (todo = "lucide/list-todo") and mapping (todo = { icon = "…", color = "…" }).
• Read config.theme.icon.admonition (opened up in core PR) for icon‑only overrides.
• Collect all types across both sources and merge per type, preferring extra.admonitions.icon when present, otherwise falling back to theme.icon.admonition.

2) Generate CSS for any type (no closed set)

• Emit CSS custom properties for each icon: --md-admonition-icon--<type> with inlined SVG data.
• Emit per‑type rules for colors (background/border/title/icon) so a new type works with only config.
• Use selectors that work for both regular and collapsible blocks: :is(.admonition, details).<type>.

3) Make templates MiniJinja‑safe

• Replace list.append with namespace + list concatenation.
• Replace dict.get with in checks and direct indexing.

4) Match theme variants

• Modern: keep body tint + border (matches modern defaults).
• Classic: apply only border + title tint (no body tint), matching classic defaults and avoiding the “double layer” look.

5) Keep dist artifacts in sync

• Update both src/partials/icons.html and dist/partials/icons.html.

Files changed

• src/partials/icons.html
• dist/partials/icons.html

Behavior changes

• Any extra.admonitions.<type> now produces a styled admonition with icon/color.
• Any theme.icon.admonition.<type> now provides icon‑only customization.
• Rendering no longer fails under MiniJinja.
• Classic theme custom colors align with classic built‑ins (no body tint).

Cross‑repo context

• Companion core PR (zensical): feat: allow arbitrary admonition icon keys zensical#265
  • Opens theme.icon.admonition as a map and removes fixed defaults.

Testing

• Built a demo project with custom admonitions (modern + classic variants) and verified:
  • no template errors,
  • icons render for arbitrary types,
  • colors apply as configured,
  • classic visuals match built‑ins.

Scope notes

• This PR does not add a config‑level title registry. Custom titles still use Markdown syntax (!!! type "Title").

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[zensical]

All commits verified and signed off.

[squidfunk]

Thanks for the PR! So, this is actually the scope of PR I thought your PR zensical/zensical#265 contains. It changes behavior fundamentally. Please see my rationale in zensical/zensical#265 (comment) why we need to turn this down right now. Since all changes are made in a template partial, you can just use template overrides to get the functionality proposed here.

Thanks for your understanding.