docs: describe remote signing upgrade process

This commit adds a short guide that explains how a remote signing node
setup should be migrated from lnd v0.14.x-beta to lnd v0.15.x-beta and
adds a note to all 0.15.x release notes.

docs/release-notes/release-notes-0.15.1.md

@@ -44,6 +44,12 @@ supports the feature.](https://github.com/lightningnetwork/lnd/pull/6633)
 The [wallet also creates P2TR change addresses by
 default](https://github.com/lightningnetwork/lnd/pull/6810) in most cases.
 
+**NOTE** for users running a remote signing setup: A manual account import is
+necessary when upgrading from `lnd v0.14.x-beta` to `lnd v0.15.x-beta`, see [the
+remote signing documentation for more
+details](../remote-signing.md#migrating-a-remote-signing-setup-from-014x-to-015x).
+Please upgrade to `lnd v0.15.2-beta` or later directly!
+
 ## `lncli`
 
 * [Add `payment_addr` flag to