Documentation: Add lemp9 to toctree

Fix "WARNING: document isn't included in any toctree" by adding
the lemp9 to mainboard/index.md.

Change-Id: Id6d8f9e2aab6dc7ad4baf1b37d88e531acb757f4
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/39800
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
diff --git a/Documentation/mainboard/index.md b/Documentation/mainboard/index.md
index f319edb..99e5dd0 100644
--- a/Documentation/mainboard/index.md
+++ b/Documentation/mainboard/index.md
@@ -144,6 +144,10 @@
 - [X11 LGA1151 series](supermicro/x11-lga1151-series/x11-lga1151-series.md)
 - [Flashing using the BMC](supermicro/flashing_on_vendorbmc.md)
 
+## System76
+
+- [Lemur Pro](system76/lemp9.md)
+
 ## UP
 
 - [Squared](up/squared/index.md)