docs(design-tokens): add foundations README (#29285)

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## What is the new behavior?
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- Added a new README.md file under core/src/foudantions, with
documentation about Design Tokens.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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---------

Co-authored-by: Maria Hutt <thetaPC@users.noreply.github.com>

core/src/foundations/README.md

@@ -0,0 +1,100 @@
+# Ionic Design Tokens
+
+## What are Design Tokens?
+
+Design Tokens are an abstract and universal language to store Design System values, creating a single source of truth that is tech-agnostic and easily scalable and extendable for various outputs such as CSS, SCSS, iOS, Android, JSON, and more. Typically stored in JSON format, they facilitate seamless scalability for integration with any language or Design Tool like Figma or Sketch.
+
+Design tokens represent small, repeated design decisions that make up a design system's visual style. Tokens replace static values, like hex codes representing colors, with descriptive and intuitive names.
+
+## Design Tokens Architecture
+
+The Ionic Design Tokens are stored on `core/src/foundations/_design-tokens.json`.
+
+Currently, five categories of tokens are stored:
+
+- Colors
+- Font
+- Space
+- Border
+- Elevation
+
+Using [Style Dictionary](https://amzn.github.io/style-dictionary/), these tokens generate 3 files inside `core/src/foundations`:
+
+- `ionic.root.scss` - native CSS Custom Properties inside a `:root` element.
+- `ionic.vars.scss` - SCSS variables, based on the native ones, that are used internally on all CSS/SCSS code.
+- `ionic.utility.scss` - list of utility classes based on each Design Token.
+
+This refers, more or less, to what's known as System Tokens, which are decisions that systematize the design language for a specific theme or context.
+
+Currently, Reference Tokens and Component Tokens are not used. Instead we are defining a base architecture for system tokens, that will inform the `:root` and global SCSS variables, and then we let the SCSS architecture propagate that to the Components.
+
+## Format
+
+The latest [W3C Design Tokens Format](https://design-tokens.github.io/community-group/format/) was adhered to, this is a recent effort to normalize the nomenclature of Design Tokens.
+
+The nomenclature has the following structure: `type - type variation - modifier name - $value`
+
+Example:
+
+```json
+{
+  "color": {
+    "$type": "color",
+    "primary": {
+      "10": {
+        "$value": "#f7faff"
+      }
+    }
+  }
+}
+```
+
+The `tokens.js` script, responsible for generating CSS and SCSS variables along with utility classes, will follow the same nomenclature: `prefix - CSS property name - type variation - modifier name`.
+
+Examples:
+
+CSS Custom Property:
+
+```css
+--ionic-color-primary-10
+```
+
+SCSS Variable:
+
+```scss
+$ionic-color-primary-10
+```
+
+Utility class:
+
+```css
+.ionic-color-primary-10
+```
+
+## When to change the Design Tokens
+
+It's not expected that the `design-tokens.json` needs to be changed frequently. This represents the base of all the Ionic Design System and any change to the json file should be taken carefully, synced with the UX/UI Team and reviewed by other devs.
+
+Adding new tokens is more or less risk free. However, changing values will result in visual breaking changes and renaming tokens might cause the build to be broken.
+
+## How to change the Design Tokens
+
+Using `npm run build.tokens` will generate the `ionic.root.scss`, `ionic.vars.scss`, and `ionic.utility.scss` files with the updated values that come from the `design-tokens.json`.
+
+Design Tokens will also be generated automatically with the `npm run build` command. This ensures that if someone modifies the JSON without running the build tokens command, the changes will be applied correctly.
+
+## How to use
+
+It's very important to highlight that only the Ionic Theme supports these tokens and they should not be used for md/ios themes.
+
+Within the component scope, variables from these global tokens should always be used. There should be no hardcoded values on component scope that relate to any of the existing tokens.
+
+Example (Chip Component):
+
+```scss
+@use '../../foundations/ionic.vars.scss' as tokens;
+
+:host {
+  --background: #{tokens.$ionic-color-neutral-10};
+}
+```