fish: add page (#1858)

pages/common/fish.md

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+# fish
+
+> The Friendly Interactive SHell.
+> A command-line interpreter designed to be user firendly.
+
+- Start interactive shell:
+
+`fish`
+
+- Execute a command:
+
+`fish -c "{{command}}"`
+
+- Run commands from a file:
+
+`fish {{file.fish}}`
+
+- Check a file for syntax errors:
+
+`fish --no-execute {{file.fish}}`
+
+- Display version information and exit:
+
+`fish --version`