Revert attempt to activate the Python virtual environment in Makefile.

It breaks the build on Netlify.

Author: stevepiercy

Makefile

@@ -3,7 +3,7 @@
 
 # You can set these variables from the command line.
 SPHINXOPTS    =
-SPHINXBUILD   = ../bin/sphinx-build
+SPHINXBUILD   = sphinx-build
 PAPER         =
 DOCS_DIR      = ./docs/
 BUILDDIR      = ../_build/

docs/contributing/setup-build.md

@@ -31,12 +31,13 @@ brew install enchant
 sudo apt-get install enchant
 ```
 
-Clone the Plone Documentation repository, then create and activate a virtual environment, and install project dependencies.
+Clone the Plone Documentation repository, then create a Python virtual environment, install project dependencies, and activate the virtual environment.
 
 ```shell
 git clone https://github.com/plone/documentation.git
 cd documentation
 make build
+source bin/activate
 ```