Add CSS type number.

docs/css_types/number.md

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+# <number>
+
+The `<number>` CSS type represents a real number, which can be an integer or a number with a decimal part (akin to a `float` in Python).
+
+## Syntax
+
+--8<-- "docs/snippets/type_syntax/number.md"
+
+## Examples
+
+### CSS
+
+```sass
+* {
+    css-rule: 6       /* Integers are numbers */
+    css-rule: -13     /* Numbers can be negative */
+    css-rule: 4.75    /* Numbers can have a decimal part */
+    css-rule: -73.73
+}
+```
+
+### Python
+
+In Python, a rule that expects a CSS type `<number>` will accept an `int` or a `float`:
+
+```py
+number = 6       # ints are numbers
+number = -13     # ints can be negative
+number = 4.75    # floats are numbers
+number = -73.73  # negative floats too
+```

docs/snippets/type_syntax/number.md

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+A [`<number>`](/css_types/number) is an [`<integer>`](/css_types/integer), optionally followed by the decimal point `.` and a decimal part composed of one or more digits.