xcode-select: add page (#3826)

Co-Authored-By: Owen Voke <owzie123@gmail.com>
Co-authored-by: Zlatan Vasović <zlatanvasovic@gmail.com>

pages/osx/xcode-select.md

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+# xcode-select
+
+> Switch between different versions of Xcode and the included developer tools.
+> Also used to update the path to Xcode if it is moved after installation.
+
+- Install Xcode's command-line tools:
+
+`xcode-select --install`
+
+- Select a given path as the active developer directory:
+
+`xcode-select -s {{path/to/Xcode.app/Contents/Developer}}`
+
+- Select a given Xcode instance and use its developer directory as the active one:
+
+`xcode-select -s {{path/to/Xcode.app}}`
+
+- Print the currently selected developer directory:
+
+`xcode-select -p`
+
+- Discard any user-specified developer directory so that it will be found via the default search mechanism:
+
+`sudo xcode-select -r`