Add "New Equation" starter template

Simple starter template with just two terms (a + b) to help users
understand the format and create their own equations.

Features:
- Minimal equation: \mark[a]{a} + \mark[b]{b}
- Self-descriptive instructions in definitions
- Shows syntax patterns for marking terms and referencing them
- Appears first in equation selector for easy discovery

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: stared

public/examples/equations.json

@@ -1,4 +1,10 @@
 [
+  {
+    "id": "new",
+    "title": "New Equation",
+    "category": "Template",
+    "file": "new.md"
+  },
   {
     "id": "schrodinger",
     "title": "Schrödinger Equation",

public/examples/new.md

@@ -0,0 +1,23 @@
+# New Equation
+
+## Equation
+
+$$
+\mark[a]{a} + \mark[b]{b}
+$$
+
+## Description
+
+[First term]{.a} plus [second term]{.b}.
+
+## .a
+
+The first term in the equation.
+
+Edit this equation to create your own!
+
+## .b
+
+The second term in the equation.
+
+Try adding more terms using `\mark[name]{content}` in the equation, then reference them with `[text]{.name}` in the description.