docs(website): contributing for Windows (#140)

www/CONTRIBUTING.md

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+# Contributing to the Ergo‑L website
+
+## Setup
+
+### For Windows
+
+* Open a terminal, for example Windows PowerShell
+* Install [Hugo](https://gohugo.io/installation/windows/) :
+  ```powershell
+  winget install Hugo.Hugo.Extended
+  ```
+* Optional if you run Hugo in a Linux terminal such as WSL or Git Bash ; install [PowerShell](https://learn.microsoft.com/en-us/powershell/scripting/install/installing-powershell-on-windows?view=powershell-7.4) (Hugo can’t run in preinstalled Windows PowerShell) :
+  ```powershell
+  winget install --id Microsoft.Powershell --source winget
+  ```
+  Note : PowerShell is not [the same application](https://learn.microsoft.com/en-us/powershell/scripting/whats-new/differences-from-windows-powershell?view=powershell-7.3) as _Windows_ PowerShell even if the name is almost the same and they are both maintained by Microsoft
+* Install [Pandoc](https://pandoc.org/installing.html#windows) :
+  ```powershell
+  winget install --source winget --exact --id JohnMacFarlane.Pandoc
+  ```
+* Launch the newly installed PowerShell (or restart it if you already had it) and go to the `www` folder :
+  ```powershell
+  cd www
+  ```
+* Run the development server :
+  ```powershell
+  hugo server
+  ```
+* And go to http://localhost:1313/ to see the effect of your changes in livereload