Base: Add man page for blockdev utility

Author: https://github.com/Granddave
Commit: 7269ce15fc
Pull-request: https://github.com/SerenityOS/serenity/pull/10401
Reviewed-by: https://github.com/supercomputer7

Base/usr/share/man/man8/blockdev.md

@@ -0,0 +1,30 @@
+## Name
+
+blockdev - query block devices
+
+## Synopsis
+
+```**sh
+$ blockdev [options] <device>
+```
+
+## Description
+
+The `blockdev` command call ioctls on the given block device.
+
+## Options
+
+* `-s`, `--size`: Get disk size in bytes
+* `-b`, `--block-size`: Get block size in bytes
+
+## Examples
+
+```sh
+# Get disk size
+# blockdev -s /dev/hda
+863718912
+
+# Get block size
+# blockdev -b /dev/hda
+512
+```