fast: add page (#5342)

pages/common/fast.md

@@ -0,0 +1,16 @@
+# fast
+
+> Test your download and upload speed using fast.com.
+> More information: <https://github.com/sindresorhus/fast-cli>.
+
+- Measure the current download speed:
+
+`fast`
+
+- Measure the current upload speed in addition to download speed:
+
+`fast --upload`
+
+- Display results on a single line to reduce spacing:
+
+`fast --single-line`