exenv: add page (#4992)

pages/common/exenv.md

@@ -0,0 +1,24 @@
+# exenv
+
+> A tool to easily install Elixir versions and manage application environments.
+> More information: <https://github.com/exenv/exenv>.
+
+- Display a list of installed versions:
+
+`exenv versions`
+
+- Use a specific version of Elixir across the whole system:
+
+`exenv global {{version}}`
+
+- Use a specific version of Elixir for the current application/project directory:
+
+`exenv local {{version}}`
+
+- Show the currently selected Elixir version:
+
+`exenv {{version}}`
+
+- Install a version of Elixir (requires `elixir-build` plugin <https://github.com/mururu/elixir-build>):
+
+`exenv install {{version}}`