Update install instruction (#3)

Summary:

Update install command with pixi task, which is simpler

Differential Revision: D58612312

README.md

@@ -30,10 +30,10 @@ Complete the following steps only once:
   cd momentum
   ```
 
-2. Build and install Momentum in a virtual environment using Pixi:
+2. Build and install Momentum in a virtual environment using Pixi (initial setup may take a few minutes):
 
   ```
-  pixi run cmake --build build --target install --parallel
+  pixi run install
   ```
 
 ### Creating Your First Application

momentum/website/docs/02_user_guide/01_getting_started.md

@@ -23,10 +23,10 @@ Complete the following steps only once:
   cd momentum
   ```
 
-2. Build and install Momentum in a virtual environment using Pixi:
+2. Build and install Momentum in a virtual environment using Pixi (initial setup may take a few minutes):
 
   ```
-  pixi run cmake --build build --target install --parallel
+  pixi run install
   ```
 
 ## Creating Your First Application