Switch to installing collection inside tox target

Instead of working around with symbolic links, we can simply just
do the installation instead inside the tox target.

Sem-Ver: bugfix
Change-Id: Ie46b6ea0aed75f512b704e4892ab07d71db62d1a
diff --git a/doc/requirements.txt b/doc/requirements.txt
index 5a1ebd7..e796768 100644
--- a/doc/requirements.txt
+++ b/doc/requirements.txt
@@ -1,3 +1,4 @@
+ansible-core
 sphinx
 sphinx_rtd_theme
 reno[sphinx]