Use ansible-documentation repo for core porting guide

As of ansible-community/community-topics#240,
the ansible documentation (including porting guides) are stored in a new
repository.

changelogs/fragments/540.yaml

@@ -0,0 +1,5 @@
+---
+bugfixes:
+  - Retrieve the ansible-core Porting Guide from the ansible-documentation
+    repo. These files are being removed from the ansible-core repo
+    (https://github.com/ansible-community/antsibull/pull/540).

src/antsibull/changelog.py

@@ -33,6 +33,7 @@
 from semantic_version import Version as SemVer
 
 from antsibull.collection_meta import CollectionsMetadata
+from antsibull.constants import CORE_PORTING_GUIDE_BASE_URL
 
 
 class ChangelogData:
@@ -320,9 +321,9 @@ async def download_changelog(
         self.changelog = ChangelogData.concatenate(changelogs)
 
     async def download_porting_guide(self, aio_session: aiohttp.client.ClientSession):
-        branch_url = "https://raw.githubusercontent.com/ansible/ansible/devel"
-
-        query_url = f"{branch_url}/{get_porting_guide_filename(self.latest)}"
+        query_url = (
+            f"{CORE_PORTING_GUIDE_BASE_URL}/{get_porting_guide_filename(self.latest)}"
+        )
         async with aio_session.get(query_url) as response:
             self.porting_guide = await response.read()
 

src/antsibull/constants.py

@@ -22,3 +22,7 @@
 DOCSITE_COMMUNITY_URL = "https://docs.ansible.com/ansible/latest/community"
 
 COLLECTION_EXCLUDE_DIRS = ("docs", "tests")
+
+CORE_PORTING_GUIDE_BASE_URL = (
+    "https://raw.githubusercontent.com/ansible/ansible-documentation/devel"
+)