btm: add page (#4300)

btm is a cross-platform graphical process/system monitor with a
customizable interface and a multitude of features.

It supports Linux, macOS, and Windows.
Inspired by both gtop and gotop.

More info: https://github.com/ClementTsang/bottom

pages/common/btm.md

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+# btm
+
+> An alternative to `top`.
+> Aims to be lightweight, cross-platform and more graphical than `top`.
+> More information: <https://github.com/ClementTsang/bottom>.
+
+- Show the default layout (cpu, memory, temperatures, disk, network, and processes):
+
+`btm`
+
+- Enable basic mode, removing charts and condensing data (similar to `top`):
+
+`btm --basic`
+
+- Use big dots instead of small ones in charts:
+
+`btm --dot_marker`
+
+- Show also battery charge and health status:
+
+`btm --battery`
+
+- Refresh every 250 milliseconds and show the last 30 seconds in the charts:
+
+`btm --rate 250 --default_time_value 30000`