chore(markdown): Update option docs

markdown/dev/reference/api/config/options/const/en.md

@@ -0,0 +1,37 @@
+---
+title: constant
+---
+
+If your option is a scalar value (like a string or a number), 
+it will be treated as a constant. Constant options are never 
+exposed in the frontend, but can still be set when using FreeSewing
+via the API.
+
+## Structure
+
+Any option holding a scalar value is a constant option.
+
+## Example
+
+```js
+options: {
+  collarFactor: 4.8,
+  fitCollar: false,
+}
+```
+
+<Tip>
+
+##### Why would you use this?
+
+There are typically two use-cases for constant options:
+
+- Rather than define constants in your code, it's good practice to set 
+them in your configuration file.  This way, people who extend your 
+pattern can change them if they would like to.
+- A constant option can be used as a feature-flag. Enabling or disabling
+parts of the code beyond the control of the end user, but accessible to
+developers.
+
+</Tip>
+