docs: Change yarn to pnpm

docs/intro/index.md

@@ -256,10 +256,10 @@ Latest Version: <https://unpkg.com/browse/mermaid@8.8.0/>
 To Deploy Mermaid:
 
 1.  You will need to install node v16, which would have npm
-2.  Download yarn using npm
-3.  Enter the following command: `yarn add mermaid`
-4.  You can then add mermaid as a dev dependency using this command:
-    `yarn add --dev mermaid`
+2.  Install mermaid
+    - NPM: `npm i mermaid`
+    - Yarn: `yarn add mermaid`
+    - Pnpm: `pnpm add mermaid`
 
 ### [Mermaid API](../config/setup/README):
 
@@ -300,22 +300,26 @@ Don't hesitate to contact me if you want to get involved!
 
 ## For contributors
 
-### Setup
+### Requirements
 
-```sh
-yarn install
-```
+- [volta](https://volta.sh/) to manage node versions.
+- [Node.js](https://nodejs.org/en/). `volta install node`
+- [pnpm](https://pnpm.io/) package manager. `volta install pnpm`
 
-### Build
+## Development Installation
 
-```sh
-yarn build:watch
+```bash
+git clone git@github.com:mermaid-js/mermaid.git
+cd mermaid
+# npx is required for first install as volta support for pnpm is not added yet.
+npx pnpm install
+pnpm test
 ```
 
 ### Lint
 
 ```sh
-yarn lint
+pnpm lint
 ```
 
 We use [eslint](https://eslint.org/).
@@ -324,7 +328,7 @@ We recommend you to install [editor plugins](https://eslint.org/docs/user-guide/
 ### Test
 
 ```sh
-yarn test
+pnpm test
 ```
 
 Manual test in browser: open `dist/index.html`