docs: Regenerate all manpages using mmark (#2762)

Mmark recently became able to create manual pages. This removed the
dependency on 'ronn' and just uses mmark (Go program).

Re-hookup Makefile.doc to generate the correct header mmark needs to
see and regenate them all.

Spot checking a few pages suggest they look good and actually better
than rendered with ronn, esp. lists in lists.

Fixes #2757

Signed-off-by: Miek Gieben <miek@miek.nl>

corefile.5.md

@@ -57,7 +57,7 @@ You can use the `import` "plugin" to include parts of other files, see <https://
 
 If you want to reuse a snippet you can define one with and then use it with *import*.
 
-~~~ corefile {
+~~~ corefile
 (mysnippet) {
     log
     whoami
@@ -73,7 +73,7 @@ If you want to reuse a snippet you can define one with and then use it with *imp
 The **ZONE** is root zone `.`, the **PLUGIN** is chaos. The chaos plugin takes an argument:
 `CoreDNS-001`. This text is returned on a CH class query: `dig CH txt version.bind @localhost`.
 
-~~~ Corefile
+~~~ corefile
 . {
    chaos CoreDNS-001
 }
@@ -82,7 +82,7 @@ The **ZONE** is root zone `.`, the **PLUGIN** is chaos. The chaos plugin takes a
 When defining a new zone, you either create a new server, or add it to an existing one. Here we
 define one server that handles two zones; that potentially chain different plugins:
 
-~~~ Corefile
+~~~ corefile
 example.org {
     whoami
 }
@@ -93,7 +93,7 @@ org {
 
 Is identical to:
 
-~~~ Corefile
+~~~ corefile
 example.org org {
     whoami
 }
@@ -101,7 +101,7 @@ example.org org {
 
 Reverse zones can be specified as domain names:
 
-~~~ Corefile
+~~~ corefile
 0.0.10.in-addr.arpa {
     whoami
 }
@@ -109,7 +109,7 @@ Reverse zones can be specified as domain names:
 
 or by just using the CIDR notation:
 
-~~~ Corefile
+~~~ corefile
 10.0.0.0/24 {
     whoami
 }
@@ -117,7 +117,7 @@ or by just using the CIDR notation:
 
 This also works on a non octet boundary:
 
-~~~ Corefile
+~~~ corefile
 10.0.0.0/27 {
     whoami
 }