docs: Add example recording rule for node_memory_MemAvailable
docs: Add example recording rule for node_memory_MemAvailable

Signed-off-by: Nicholas Capo <59161f3fe1f02079fd44f8029abbaa24935e030d@gmail.com>

siavashs/node_exporter,prometheus/node_exporter,derekmarcotte/node_exporter,derekmarcotte/node_exporter,siavashs/node_exporter,derekmarcotte/node_exporter,siavashs/node_exporter,derekmarcotte/node_exporter,prometheus/node_exporter,siavashs/node_exporter,prometheus/node_exporter