Improve upgrade preparation.

- Change indentation levels of the first list.
- Point to dist.plone.org instead of CMFPlone for the release notes.
- Remove instructions on first installing a newer Zope version. This was from the days before Zope and Plone were released as eggs.
- Add links to Zope documentation.
- Do not tell people to look at README/INSTALL/UPGRADE.txt in CMFPlone. I never update these.
- Fix backup link.

Author: mauritsvanrees

docs/backend/upgrading/preparations.md

@@ -16,54 +16,45 @@ Things to do before you migrate Plone.
 ## Gather Information
 
 - Read the "What's new in..." for your relevant Plone version, and read the release notes.
-
-  - You'll find these in the CMFPlone directory of the distribution of the new version of Plone.
+  You'll find these on `dist.plone.org` in the release directory of the new version of Plone.
+  As an example, here are the [6.0.0b2 release notes](https://dist.plone.org/release/6.0.0b2/RELEASE-NOTES.md).
 
 - Make sure to check installed add-ons, if you **do not** need certain add-ons anymore, please deactivate and deinstall.
 
-- Check for dependencies
+- Check for dependencies.
 
   - Read the release notes for the Plone release you are upgrading to, in particular:
 
-  - What version of Python is required?
+    - What version of Python is required?
 
-  - What version of Zope is required?
+    - What version of Zope is required?
 
-  - Do you need any new Python libraries?
+    - Do you need any new Python libraries?
 
   - Make sure all the add-on products you are using have updated to support the version of Plone you are upgrading to.
 
   - Start with the third-party products that are in use on your site.
+    Verify that they have been updated or verified to work on the new version, and get them upgraded in your existing instance before you start the Plone/Zope/Python upgrade if possible.
 
-    - Verify that they have been updated or verified to work on the new version, and get them upgraded in your existing instance before you start the Plone/Zope/Python upgrade if possible.
-
-  - If Zope depends on a newer version of Python, install the new version of Python first.
-
-  - If the newer version of Plone depends on a newer version of Zope, you will need to install that before proceeding with the Plone upgrade.
 
 ```{note}
-Zope has its own migration guidelines, which you will find in the release notes of the version you are migrating to.
+Zope has its own documentation.
+Of most interest for migration are:
+
+- [What's new](https://zope.readthedocs.io/en/latest/news.html) on a high level.
+- [Migrating between Zope versions](https://zope.readthedocs.io/en/latest/migrations/index.html).
+- [Detailed changelog](https://zope.readthedocs.io/en/latest/changes.html).
 
 If Plone is being upgraded at the same time as a Zope version, Plone will usually handle the Zope upgrade with its own migration script.
 ```
 
-- Read the following files in the CMFPlone directory of the distribution of the new version of Plone you want to update to:
-
-  - README.txt
-
-  - INSTALL.txt
-
-  - UPGRADE.txt (although this usually contains only the general procedure outlined above)
-
-    - These files are important because they may contain important last minute information and might be more specific than the relevant sections of this reference manual.
-
 ## Back Up Your Plone Site
 
 ```{note}
 It's important to back up your Plone site.
 ```
 
-You will find a {doc}`how-to on backing up your Plone site here </manage/deploying/backup>`.
+You will find a {doc}`how-to on backing up your Plone site here </deployment/backup>`.
 
 ## Setup A Test Environment To Rehearse The Upgrade