Add section headers to ingredient and direction lists

[edinel]

What

Lets a recipe group its ingredients and directions under section headers
(e.g. "For the sofrito", "For the paella") for recipes with distinct
sub-preparations.

An ingredients or directions entry whose text begins with # renders
as a styled heading instead of a list item:

ingredients:
  - "# For the sofrito"
  - "1/4 cup extra-virgin olive oil"
  - "# For the paella"
  - "3/4 cup Bomba rice"

Details

• Purely additive: flat string lists are untouched, so existing recipes
  render exactly as before.
• Header <li>s carry no itemprop, so they're excluded from the
  schema.org recipeIngredient / recipeInstructions data.
• Header <li>s are skipped by the click-to-check script (no checkbox,
  no scroll toggle).
• Not applied to the component-recipe loops in this PR; the same
  three-line guard could be added there too.

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