pio-system: add page (#5523)

pages/common/pio-system.md

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+# pio system
+
+> Miscellaneous system commands for PlatformIO.
+> More information: <https://docs.platformio.org/en/latest/core/userguide/system/index.html>.
+
+- Install shell completion for the current shell (supports bash, fish, zsh and powershell):
+
+`pio system completion install`
+
+- Uninstall shell completion for the current shell:
+
+`pio system completion uninstall`
+
+- Display system-wide PlatformIO information:
+
+`pio system info`
+
+- Remove unused PlatformIO data:
+
+`pio system prune`
+
+- Remove only cached data:
+
+`pio system prune --cache`
+
+- List unused PlatformIO data that would be removed but do not actually remove it:
+
+`pio system prune --dry-run`