devenv: add page (#12862)

Co-authored-by: Fazle Arefin <fazlearefin@users.noreply.github.com>
Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>
Co-authored-by: spageektti <git@spageektti.cc>

pages/common/devenv.md

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+# devenv
+
+> Fast, Declarative, Reproducible and Composable Developer Environments using Nix.
+> More information: <https://devenv.sh>.
+
+- Initialise the environment:
+
+`devenv init`
+
+- Enter the Development Environment with relaxed hermeticity (state of being airtight):
+
+`devenv shell --impure`
+
+- Get detailed information about the current environment:
+
+`devenv info --verbose`
+
+- Start processes with `devenv`:
+
+`devenv up --config /{{file}}/{{path}}/`
+
+- Clean the environment variables and re-enter the shell in offline mode:
+
+`devenv --clean --offline`
+
+- Delete the previous shell generations:
+
+`devenv gc`