Docs: Rearrange TF-M building instructions

- move the building instructios to a dedicated folder, reflecting main TOC
- adjust links in the documentation

Signed-off-by: Anton Komlev <anton.komlev@arm.com>
Change-Id: I335de95061938d619a47cd813020121809e1459b
diff --git a/docs/index.rst b/docs/index.rst
index 8fc23ff..add4ec2 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -86,7 +86,7 @@
     :hidden:
 
     Platform Selection<platform/ext/index>
-    Building <technical_references/instructions/tfm_build_instruction>
+    building/tfm_build_instruction
     integration_guide/index
     technical_references/design_docs/index
     Tools <tools/index>