Documentation/lenovo: Replace RST code with markdown

Latest Sphinx supports up path traversal in markdown. Replace old
RST code that's no longer needed to prevent it being copy and pasted.

Change-Id: Ieec5cc1f8d91a7fbc003efae465f61e6b72b39dc
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/38930
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Peter Lemenkov <lemenkov@gmail.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

Documentation/mainboard/lenovo/x301.md

@@ -26,9 +26,8 @@ The vendor IFD VSCC list contains:
     -WINBOND_NEX_W25X64 (0xef, 0x3017)
     -ATMEL_AT25DF641 (0x1f, 0x4800)
 
-```eval_rst
-:doc:`../../flash_tutorial/ext_power`
-```
+The general [flashing tutorial] has more details.
+
 Tested:
     - CPU Core 2 Duo U9400
     - Slotted DIMM 4GiB*2 from samsung
@@ -42,3 +41,7 @@ Tested:
     - S3
     - Linux 4.19.67-2 within Debian GNU/Linux stable, loaded from
       Linux payload (Heads) and Seabios.
+
+
+[flashing tutorial]: ../../flash_tutorial/ext_power.md
+