Resolve conflicts with 6-dev in conf.py

Author: stevepiercy

docs/conf.py

@@ -81,6 +81,7 @@
     # Ignore specific anchors
     r"https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS/Errors#Identifying_the_issue",
     r"https://github.com/browserslist/browserslist#queries",
+    r"https://github.com/plone/cookiecutter-zope-instance#options",
     r"https://github.com/plone/plone.docker#for-basic-usage",
     r"https://github.com/plone/plone.rest#cors",
     r"https://github.com/plone/plone.volto/blob/6f5382c74f668935527e962490b81cb72bf3bc94/src/kitconcept/volto/upgrades.py#L6-L54",
@@ -280,3 +281,21 @@
 html_context = {
     "edit_page_url_template": "https://6.dev-docs.plone.org/contributing/index.html?{{ file_name }}#making-contributions-on-github",
 }
+
+# An extension that allows replacements for code blocks that
+# are not supported in `rst_epilog` or other substitutions.
+# https://stackoverflow.com/a/56328457/2214933
+def source_replace(app, docname, source):
+    result = source[0]
+    for key in app.config.source_replacements:
+        result = result.replace(key, app.config.source_replacements[key])
+    source[0] = result
+
+# Dict of replacements.
+source_replacements = {
+    "{PLONE_BACKEND_VERSION}": "6.0.0b2",
+}
+
+def setup(app):
+   app.add_config_value('source_replacements', {}, True)
+   app.connect('source-read', source_replace)