docs(web-components): add notes on TypeScript definition files (#5044)

### Related Ticket(s)

Refs #4988.

### Description

Adds implementation notes for TypeScript definition files in `@carbon/ibmdotcom-web-components` so that new developers can quickly understand the difference from regular NPM packages.

### Changelog

**New**

- Implementation notes for TypeScript definition files in `@carbon/ibmdotcom-web-components`.

packages/web-components/IMPLEMENTATION_NOTES.md

@@ -25,6 +25,7 @@
 - [Container components](#container-components)
   - [Triggering action dispatcher](#triggering-action-dispatcher)
 - [Masthead search](#masthead-search)
+- [TypeScript type definition](#typescript-type-definition)
 - [Vendor directory](#vendor-directory)
 - [RTL support](#rtl-support)
 
@@ -258,6 +259,14 @@ The search box feature in masthead consists of two things:
 
 `<dds-masthead-search-container>` [intersects the search query with the table of loaded search results](https://github.com/carbon-design-system/carbon-for-ibm-dotcom/blob/v1.13.0/packages/web-components/src/components/masthead/masthead-search-container.ts#L64-L70) and [sets `currentSearchResults` property](https://github.com/carbon-design-system/carbon-for-ibm-dotcom/blob/v1.13.0/packages/web-components/src/components/masthead/masthead-search-container.ts#L72), which is [rendered as `<dds-masthead-search-item>`](https://github.com/carbon-design-system/carbon-for-ibm-dotcom/blob/v1.13.0/packages/web-components/src/components/masthead/masthead-search-composite.ts#L118-L123).
 
+## TypeScript type definition
+
+All TypeScript code in our source code is transpiled to plain JavaScript code, by a [Babel plugin](https://www.npmjs.com/package/@babel/plugin-transform-typescript). Applications can still refer to our TypeScript types, via `.d.ts` files we ship.
+
+The `.d.ts` files are generated by [regular TypeScript toolchain](https://www.npmjs.com/package/typescript), via [`gulp-typescript`](https://www.npmjs.com/package/gulp-typescript). The build procedure can be found at [here](https://github.com/carbon-design-system/carbon-for-ibm-dotcom/blob/v1.15.0/packages/web-components/gulp-tasks/build.js#L474-L490).
+
+> 💡 The `.d.ts` for React wrappers are generated by our [custom Babel plugin](https://github.com/carbon-design-system/carbon-for-ibm-dotcom/blob/v1.15.0/packages/web-components/tools/babel-plugin-create-react-custom-element-type-def.js). An example of such `.d.ts` file can be found [here](https://unpkg.com/@carbon/[email protected]/es/components-react/card/card.d.ts). The build procedure can be found [here](https://github.com/carbon-design-system/carbon-for-ibm-dotcom/blob/v1.15.0/packages/web-components/gulp-tasks/build.js#L388-L397).
+
 ## Vendor directory
 
 `@carbon/ibmdotcom-services-store` is a private package that is not published to NPM. Putting it to `package.json` will break application because it won't be found.