Generated docs if hotdoc is installed.

run_tests.py

diff --git a/run_tests.py b/run_tests.py
index d0a67e8..4ef0cd1 100755
--- a/run_tests.py
+++ b/run_tests.py
@@ -127,6 +127,8 @@
     # Running on a developer machine? Be nice!
     if not mesonlib.is_windows() and 'TRAVIS' not in os.environ:
         os.nice(20)
+    if shutil.which('hotdoc'):
+        returncode += subprocess.call(['hotdoc', 'run', '--disable-incremental'], cwd='docs')
     # Appveyor sets the `platform` environment variable which completely messes
     # up building with the vs2010 and vs2015 backends.
     #