coreutils commands: move pages to common/ folder (#2442)

pages/common/b2sum.md

@@ -0,0 +1,19 @@
+# b2sum
+
+> Calculate BLAKE2 cryptographic checksums.
+
+- Calculate the BLAKE2 checksum for a file:
+
+`b2sum {{filename1}}`
+
+- Calculate BLAKE2 checksums for multiple files:
+
+`b2sum {{filename1}} {{filename2}}`
+
+- Read a file of BLAKE2 sums and filenames and verify all files have matching checksums:
+
+`b2sum -c {{filename.b2}}`
+
+- Calculate the BLAKE2 checksum from stdin:
+
+`{{somecommand}} | shasum`