docs(misc) correct and enhance npm install snippets (#3316)

Author: EugeneHlushko

src/content/guides/getting-started.md

@@ -33,8 +33,7 @@ First let's create a directory, initialize npm, [install webpack locally](/guide
 mkdir webpack-demo
 cd webpack-demo
 npm init -y
-npm install webpack --save-dev
-npm install webpack-cli --save-dev
+npm install webpack webpack-cli --save-dev
 ```
 
 T> Throughout the Guides we will use `diff` blocks to show you what changes we're making to directories, files, and code.

src/content/guides/installation.md

@@ -27,6 +27,7 @@ To install the latest release or a specific version, run one of the following co
 
 ``` bash
 npm install --save-dev webpack
+# or specific version
 npm install --save-dev webpack@<version>
 ```
 
@@ -64,6 +65,7 @@ If you are enthusiastic about using the latest that webpack has to offer, you ca
 
 ``` bash
 npm install webpack@beta
+# or specific tag/branch
 npm install webpack/webpack#<tagname/branchname>
 ```