Added Akka.NET v1.4 upgrade advisories area to website (akkadotnet#5289)

* added upgrade advisories section for Akka.NET website

* cleaned up formatting and navigation

* reconciled changes with akkadotnet#5290

* fixed `double` and `float` references since they're not handled by this serializer

docs/community/whats-new/akkadotnet-v1.4-upgrade-advisories.md

@@ -0,0 +1,67 @@
+---
+uid: akkadotnet-v14-upgrade-advisories
+title: Akka.NET v1.4 Upgrade Advisories
+---
+
+# Akka.NET v1.4 Upgrade Advisories
+This document contains specific upgrade suggestions, warnings, and notices that you will want to pay attention to when upgrading between versions within the Akka.NET v1.4 roadmap.
+
+## Upgrading to Akka.NET v1.4.20 from Older Versions
+
+> [!NOTE]
+> This is an edge-case issue that only affects users who are sending primitive data types (i.e. `int`, `long`, `string`) directly over Akka.Remote or Akka.Persistence.
+
+Between Akka.NET v1.4.19 and [v1.4.20](https://github.com/akkadotnet/akka.net/releases/tag/1.4.20) we introduced a regression in the wire format of the "primitive" serializer in Akka.NET v1.4.20 - the serializer responsible for transmitting `int`, `long`, and `string` as stand-alone messages in Akka.Remote and Akka.Persistence.
+
+The error message would look like this in clusters that are running a combination of v1.4.20 and any previous versions of Akka.NET:
+
+> `Serializer not defined for message with serializer id [6] and manifest []. Transient association error (association remains live). Cannot find manifest class [S] for serializer with id [17].`
+
+You can see the PR that introduced this regression here: https://github.com/akkadotnet/akka.net/issues/4986
+
+This change was originally introduced to assist with cross-platform wire compatibility between .NET Framework and .NET Core, because Microsoft changed the names of all primitive types between the two runtimes when .NET Core was originally introduced.
+
+**Workarounds**:
+
+To work around this issue, if you're affected by it (most users are not:)
+
+* Upgrade all of the nodes to v1.4.20 or later at once;
+* Or upgrade directly to Akka.NET v1.4.26 or later, which resolves this issue and prevents the regression from occurring.
+
+## Upgrading to Akka.NET v1.4.26 from Older Versions
+
+> [!NOTE]
+> This is an edge-case issue that only affects users who are sending primitive data types (i.e. `int`, `long`, `string`) directly over Akka.Remote or Akka.Persistence.
+
+In Akka.NET v1.4.26 we have introduced a new setting:
+
+```
+akka.actor.serialization-settings.primitive.use-legacy-behavior = on
+```
+
+This setting is set of `on` by default and it resolves the backwards compatibility issue introduced in the "primitives" serializer described in our [v1.4.20 upgrade advisory](#upgrading-to-akkanet-v1420-from-older-versions).
+
+> [!IMPORTANT]
+> If you have: 
+> * Previously upgraded to Akka.NET v1.4.20+ and you have not run into any issues;
+> * You have not yet upgraded to Akka.NET v1.4.20+; and
+> * You _do not_ plan on running both .NET Framework and .NET Core in the same cluster
+> Then you can safely upgrade to v1.4.26 using your normal deployment process. 
+
+If you are running a mixed .NET Core and .NET Framework cluster, see the process below.
+
+### Deploying v1.4.26 into Mixed .NET Core and .NET Framework Environments
+*However*, if you are attempting to run a mixed-mode cluster - i.e. some services running on .NET Framework and some running on .NET Core, you will eventually want to turn this setting to `off` in order to faciliate smooth operation between both platforms.
+
+#### Already Deployed v1.4.20 or Later
+If you've already deployed v1.4.20 and you have not had any issues with the primitives serializer, do the following:
+
+1. Before you upgrade to v1.4.26 or later set `akka.actor.serialization-settings.primitive.use-legacy-behavior = off` - so any future serialization of primitives will be handled correctly in a cross-platform way;
+2. Run your normal deployment process.
+
+#### Have Not Deployed v1.4.20 or Later
+If you have not previously deployed to v1.4.20 or later, then do the following:
+
+1. Deploy once with `akka.actor.serialization-settings.primitive.use-legacy-behavior = on` (the default);
+2. Once all nodes are completely upgraded to v1.4.26 and later, prepare a second deployment with `akka.actor.serialization-settings.primitive.use-legacy-behavior = off`;
+3. Complete the second deployment - you will be fine with all rolling upgrades moving forward.

docs/community/whats-new/toc.yml

@@ -1,2 +1,4 @@
 - name: New in Akka.NET v1.4
-  href: akkadotnet-v1.4.md
+  href: akkadotnet-v1.4.md
+- name: Upgrade Advisories (v1.4)
+  href: akkadotnet-v1.4-upgrade-advisories.md