Move sliding-sync proxy Note to the top of its docs page

People should be made aware of the status of this component as soon as
possible.

This is consitent with the positioning of Note messages we have for other deprecated
components (e.g. bot-chatgpt, bot-go-neb, etc).

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/3735

docs/configuring-playbook-sliding-sync-proxy.md

@@ -1,5 +1,7 @@
 # Setting up the Sliding Sync proxy (optional)
 
+**Note**: The sliding-sync proxy is **not required** anymore as it's been replaced with a different method (called Simplified Sliding Sync) which is integrated into newer homeservers by default (**Conduit** homeserver from version `0.6.0` or **Synapse** from version `1.114`). This component and documentation remain here for historical purposes, but **installing this old sliding-sync proxy is generally not recommended anymore**.
+
 The playbook can install and configure [sliding-sync](https://github.com/matrix-org/sliding-sync) proxy for you.
 
 Sliding Sync is an implementation of [MSC3575](https://github.com/matrix-org/matrix-spec-proposals/blob/kegan/sync-v3/proposals/3575-sync.md) and a prerequisite for running the new (**still beta**) Element X clients ([Element X iOS](https://github.com/element-hq/element-x-ios) and [Element X Android](https://github.com/element-hq/element-x-android)).
@@ -10,7 +12,6 @@ Element X iOS is [available on TestFlight](https://testflight.apple.com/join/uZb
 
 Element X Android is [available on the Github Releases page](https://github.com/element-hq/element-x-android/releases).
 
-**Note**: The sliding-sync proxy is **not required** anymore as it's been replaced with a different method (called Simplified Sliding Sync) which is integrated into newer homeservers by default (**Conduit** homeserver from version `0.6.0` or **Synapse** from version `1.114`).
 
 ## Adjusting the playbook configuration