docs: Direct documentation build

- Stop prebuilding conf.py and passing environmental info there
- Simplify CMake script for Shpinx doc generation
- Enable true incremental build for *.rst files

Signed-off-by: Anton Komlev <anton.komlev@arm.com>
Change-Id: I0c49043cda43dca263f530dde13052bcf9f49046
diff --git a/docs/contributing/lic.rst b/docs/contributing/lic.rst
index b8d4ce5..2beec63 100644
--- a/docs/contributing/lic.rst
+++ b/docs/contributing/lic.rst
@@ -3,7 +3,7 @@
 License
 #######
 
-.. include:: /license.rst
+.. include:: ../../license.rst
 
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