Docs: Refine tfm_sw_requirement.rst

Simplify the tfm software requirement document and change the document
to 'tfm_getting_started.rst', let it more
suitable for new starter.

Signed-off-by: Summer Qin <summer.qin@arm.com>
Change-Id: I10379aa92ac2c6cd784f0fd1b1b5431243716d74
diff --git a/docs/getting_started/index.rst b/docs/getting_started/index.rst
index 9ddb2d0..a3e729b 100644
--- a/docs/getting_started/index.rst
+++ b/docs/getting_started/index.rst
@@ -5,7 +5,7 @@
     :glob:
     :numbered:
 
-    tfm_sw_requirement
+    tfm_getting_started
 
 --------------