lein: add page (#1593)

pages/common/lein.md

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+# lein
+
+> Manage clojure projects with declarative configuration.
+
+- Generate scaffolding for a new project based on a template:
+
+`lein new {{template_name}} {{project_name}}`
+
+- Start a repl session either with the project or standalone:
+
+`lein repl`
+
+- Run the project's `-main` function with optional args:
+
+`lein run {{args}}`
+
+- Run the project's tests:
+
+`lein test`
+
+- Package up the project files and all its dependencies into a jar file:
+
+`lein uberjar`