Documentation/mainboard: Add missing Asus toctree references

The new pages for the P8Z77-M, P2B-LS, and P3B-F were missing from
index.md, causing Sphinx to output "document isn't included in any
toctree" warnings.

Change-Id: I7883d48bfbe6bff5595aa9303f9d6f4a55eadc9c
Signed-off-by: Nicholas Chin <nic.c3.14@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/74189
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
diff --git a/Documentation/mainboard/index.md b/Documentation/mainboard/index.md
index 272c6e0..c2405de 100644
--- a/Documentation/mainboard/index.md
+++ b/Documentation/mainboard/index.md
@@ -23,11 +23,14 @@
 
 - [A88XM-E](asus/a88xm-e.md)
 - [F2A85-M](asus/f2a85-m.md)
+- [P2B-LS](asus/p2b-ls.md)
+- [P3B-F](asus/p3b-f.md)
 - [P5Q](asus/p5q.md)
 - [P8C WS](asus/p8c_ws.md)
 - [P8H61-M LX](asus/p8h61-m_lx.md)
 - [P8H61-M Pro](asus/p8h61-m_pro.md)
 - [P8H77-V](asus/p8h77-v.md)
+- [P8Z77-M](asus/p8z77-m.md)
 - [P8Z77-M Pro](asus/p8z77-m_pro.md)
 - [P8Z77-V](asus/p8z77-v.md)
 - [wifigo_v1](asus/wifigo_v1.md)