Documentation/testing: Add MTD transfer rate test app docs

Add documentation entry for the `mtd` test/benchmark application.

Signed-off-by: Tiago Medicci Serrano <tiago.medicci@espressif.com>

Documentation/applications/benchmarks/mtd/index.rst

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+============================================
+``mtd`` MTD test and transfer rate benchmark
+============================================
+
+This testing/benchmark application performs an erase/write operation to
+evaluate write transfer rate and then reads the written content back to
+evaluate the read transfer rate. Finally, it compares the read data with
+the previously written data to ensure the MTD device is working as expected.
+
+EXAMPLE::
+
+  nsh> mtd /dev/mtdblock0
+  FLASH Test on device with:
+    Sector size:        4096
+    Sector count:        256
+    Erase block:        4096
+    Total size:      1048576
+
+  Starting write operation...
+
+  Write operation completed in 5.46 seconds
+  Total bytes written: 1048576
+  Transfer rate [write]: 187.55 KiB/s
+
+  Starting read operation...
+
+  Read operation completed in 0.11 seconds
+  Total bytes read: 1048576
+  Transfer rate [read]: 9309.09 KiB/s
+
+  Data verification successful: read data matches written data