docs+scripts: switch to detached signatures

Due to a misunderstanding of how the gpg command line options work, we
didn't actually create detached signatures because the --clear-sign
flag would overwrite that. We update our verification script to now only
download the detached signatures and verify them against the main
manifest file.
We also update the signing instructions.

docs/release.md

@@ -99,11 +99,11 @@ script in the image that can be called (before starting the container for
 example):
 
 ```shell
-$ docker pull lightninglabs/lnd:v0.12.0-beta
-$ docker run --rm --entrypoint="" lightninglabs/lnd:v0.12.0-beta /verify-install.sh
-$ OK=$?
-$ if [ "$OK" -ne "0" ]; then echo "Verification failed!"; exit 1; done
-$ docker run lightninglabs/lnd [command-line options]
+⛰  docker pull lightninglabs/lnd:v0.12.0-beta
+⛰  docker run --rm --entrypoint="" lightninglabs/lnd:v0.12.0-beta /verify-install.sh
+⛰  OK=$?
+⛰  if [ "$OK" -ne "0" ]; then echo "Verification failed!"; exit 1; done
+⛰  docker run lightninglabs/lnd [command-line options]
 ```
 
 # Signing an Existing Manifest File
@@ -121,8 +121,6 @@ signature during signing.
 
 Assuming `USERNAME` is your current nick as a developer, then the following
 command will generate a proper signature:
+```shell
+⛰  gpg --detach-sig --output manifest-USERNAME-TAG.sig manifest-TAG.txt
 ```
-gpg --detach-sig --output manifest-USERNAME-TAG.txt.asc --clear-sign manifest-TAG.txt
-```
-
-