Move all the docs to docs/ dir

docs/installing_plugins_via_cmd_line.md

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+# Installing plugins via the command line only
+
+### From inside tmux
+
+When you have `tmux` running, plugins can be installed with the following command:
+
+    ~/.tmux/plugins/tpm/scripts/install_plugins.sh
+
+This is the equivalent of pressing `prefix + I`.
+
+### From outside tmux
+
+To install the plugins when `tmux` is not even started (for example in a provisioning script):
+
+    # start tmux and create a session but don't attach to it
+    tmux new-session -d
+    # install the plugins
+    tmux run "~/.tmux/plugins/tpm/scripts/install_plugins.sh"
+    # killing the session is not required
+    tmux kill-session