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
10 files changed