fix: inter-book references need to be sensitive to the version of the book

A reference in the glossary was changed on master after maple, and now
maple builds fail because the reference goes to /latest/ rather than
/maple/.   This variable will let us change Maple to refer to other
maple books.

Author: Ned Batchelder

shared/conf.py

@@ -7,6 +7,13 @@
 
 import edx_theme
 
+# What release line is this?  Use "master" for master, and the release name
+# on release branches.  Zebrawood should have "zebrawood".
+release_line = "master"
+
+# The slug that is used by ReadTheDocs for this version of the projects.
+project_version = "latest" if (release_line == "master") else f"open-release-master.{release_line}"
+
 # on_rtd is whether we are on readthedocs.io, this line of code grabbed from docs.readthedocs.io
 on_rtd = os.environ.get('READTHEDOCS', None) == 'True'
 
@@ -158,7 +165,7 @@ def feedback_form_url(project, page):
 
 def edx_rtd_url(slug):
     """Use this with the readthedoc project slug to create the full URL."""
-    return f"https://edx.readthedocs.io/projects/{slug}/en/latest/"
+    return f"https://edx.readthedocs.io/projects/{slug}/en/{project_version}/"
 
 def ism_location(dir_name):
     """Calculate the intersphinx_mapping location to use for a book.
@@ -195,4 +202,3 @@ def ism_location(dir_name):
     # :jira:`TNL-4904` becomes: <a href='https://openedx.atlassian.net/browse/TNL-4904'>TNL-4904</a>
     'jira': ('https://openedx.atlassian.net/browse/%s', ''),
 }
-