Some small doc fiddles [ci skip]

* Use os.environ.copy() in examples - better not to get the
  Python proxy class that is os.environ, and instead a real dict.
* Fix spelling on a python dict method: set_default -> setdefault
* Update some wording on prepending in the user guide (where it
  still said "append to the beginning")
* Change the rather oblique note in the recently modified section
  on Append of values to CPPDEFINES with something more explicit.
* And, modify the sphinx config to get the version from SCons
  rather than hardcoding.

Fixes #3867

Signed-off-by: Mats Wichmann <mats@linux.com>

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