doc: Update gpg key fetching instructions.

This mirrors the steps performed by etc/guix-install.sh.

* doc/guix.texi (KEY-SERVER): Replace moribund SKS key servers with...
(OPENPGP-SIGNING-KEY-URL): ...the TLS PKI of the future.
(Binary Installation, USB Stick and DVD Installation): Update the
@example gpg command to download the key directly from Savannah.

doc/guix.texi

@@ -11,7 +11,7 @@
 
 @c Identifier of the OpenPGP key used to sign tarballs and such.
 @set OPENPGP-SIGNING-KEY-ID 3CE464558A84FDC69DB40CFB090B11993D9AEBB5
-@set KEY-SERVER pool.sks-keyservers.net
+@set OPENPGP-SIGNING-KEY-URL https://sv.gnu.org/people/viewgpg.php?user_id=15145
 
 @c Base URL for downloads.
 @set BASE-URL https://ftp.gnu.org/gnu/guix
@@ -558,8 +558,8 @@ If that command fails because you do not have the required public key,
 then run this command to import it:
 
 @example
-$ gpg --keyserver @value{KEY-SERVER} \
-      --recv-keys @value{OPENPGP-SIGNING-KEY-ID}
+$ wget @value{OPENPGP-SIGNING-KEY-URL} \
+      -qO - | gpg --import -
 @end example
 
 @noindent
@@ -1905,8 +1905,8 @@ If that command fails because you do not have the required public key,
 then run this command to import it:
 
 @example
-$ gpg --keyserver @value{KEY-SERVER} \
-      --recv-keys @value{OPENPGP-SIGNING-KEY-ID}
+$ wget @value{OPENPGP-SIGNING-KEY-URL} \
+      -qO - | gpg --import -
 @end example
 
 @noindent