doc: remove bitcoin PPA from install instructions, use snap

the PPA is no longer maintained, the new hotness is the snapd install.
except that snapd does some weirdness with the binary names, calling it
bitcoin-core.daemon and bitcoin-core.cli. To get around this, you can
create a symbolic link to the snap binaries, which is what we outline
here.

doc/INSTALL.md

@@ -41,12 +41,13 @@ Get dependencies:
       gettext
 
 If you don't have Bitcoin installed locally you'll need to install that
-as well:
+as well. It's now available via [snapd](https://snapcraft.io/bitcoin-core).
 
-    sudo apt-get install software-properties-common
+    sudo apt-get install snapd
-    sudo add-apt-repository ppa:bitcoin/bitcoin
+    sudo snap install bitcoin-core
-    sudo apt-get update
+    # Snap does some weird things with binary names; you'll
-    sudo apt-get install -y bitcoind
+    # want to add a link to them so everything works as expected
+    sudo ln -s /snap/bitcoin-core/current/bin/bitcoin{d,-cli} /usr/local/bin/
 
 Clone lightning: