doc: fix an error if the doc is not generated from github

The commit 57c45c0 expect Sphinx to find the environment variable
RELEASE_TAG.
It seems it's only the case when the documentation is generated from
github using the workflow defined in the release.yml file.

This commit allows to generate the doc from outside github, by setting
version to the current commit id if we are on a git repository, and
"n/a" otherwise.

doc/sphinx/conf.py

@@ -13,6 +13,7 @@
 # documentation root, use os.path.abspath to make it absolute, like shown here.
 #
 import os
+import subprocess
 # import sys
 # sys.path.insert(0, os.path.abspath('.'))
 
@@ -24,7 +25,16 @@
 author = 'SiFive Inc.'
 
 # The short X.Y version
-version = os.environ['RELEASE_TAG']
+# if github provides us a ref or tag
+if 'RELEASE_TAG' in os.environ:
+    version = os.environ['RELEASE_TAG']
+# if we are in a git repository
+elif subprocess.call(["git", "branch"], stderr=subprocess.STDOUT, stdout=open(os.devnull, 'w')) == 0:
+    # set version to the current commit id 
+    version = subprocess.check_output(['git', 'rev-parse', 'HEAD']).strip().decode('ascii')
+else:
+    version = "n/a"
+
 # The full version, including alpha/beta/rc tags
 release = version