docs: Fix /boot security hole warning in examples

The alternative would be to do this automatically if format=="vfat" and
mountpoint=="/boot", but it's better to be upfront about this.

Fixes #527

docs/HowTo.md

@@ -150,6 +150,7 @@ or with pinning:
                 type = "filesystem";
                 format = "vfat";
                 mountpoint = "/boot";
+                mountOptions = [ "umask=0077" ];
               };
             };
             root = {

docs/disko-install.md

@@ -65,6 +65,7 @@ example we assume a system that has been booted with EFI:
                       type = "filesystem";
                       format = "vfat";
                       mountpoint = "/boot";
+                      mountOptions = [ "umask=0077" ];
                     };
                   };
                   root = {
@@ -168,6 +169,7 @@ Add this to your flake.nix output:
                       type = "filesystem";
                       format = "vfat";
                       mountpoint = "/boot";
+                      mountOptions = [ "umask=0077" ];
                     };
                   };
                   root = {

docs/table-to-gpt.md

@@ -122,6 +122,7 @@ The fixed disko configuration would look like this:
             type = "filesystem";
             format = "vfat";
             mountpoint = "/boot";
+            mountOptions = [ "umask=0077" ];
           };
         };
         root = {