doc: Update building docs section

* Use python3 as a binary (most of distros still have python reserved for
  python2).
* Mention sphinx can be used from package.
* While at it, replace 'source' with '.' (shorter, although most of the
  developers use shell which support source bashism, let's not encourage
  to use it).

Link: https://lore.kernel.org/ltp/[email protected]/
Reviewed-by: Andrea Cervesato <[email protected]>
Signed-off-by: Petr Vorel <[email protected]>

Author: pevik

doc/developers/documentation.rst

@@ -21,10 +21,10 @@ command as following:
     cd doc
 
     # prepare virtual enviroment
-    python -m virtualenv .venv
-    source .venv/bin/activate
+    python3 -m virtualenv .venv
+    . .venv/bin/activate
 
-    pip install sphinx
+    pip install sphinx # usually packaged in distros
     pip install -r requirements.txt
 
     # build documentation