docs: Documentation restructure

- Change main index structure
- Sort out integration and porting guidelines
- Resort table with releases
- Add Tools section. To be adjusted further for tf-m-tools

Intention to minimise files movement to simplify review process.
Affected files shall be moved later after the new structure agreement.

Signed-off-by: Anton Komlev <anton.komlev@arm.com>
Change-Id: I0352bebabe09d4f7b5e8848ee620c0cea6881c94
diff --git a/docs/conf.py b/docs/conf.py
index 796fa83..93912b3 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -107,9 +107,11 @@
 # documentation.
 #
 html_theme_options = {
-    'collapse_navigation': False,
-    'prev_next_buttons_location' : None   # Hide Prev and Next buttons
+    'collapse_navigation' : False,
+    'prev_next_buttons_location' : None,   # Hide Prev and Next buttons
 #    'display_version': True,    # Show version under logo
+    'sticky_navigation': True,
+    'navigation_depth': 2,
 }
 
 # Remove the "View page source" link from the top of docs pages