Makefile: replace mrkd with lowdown(1).

Here's the before-vs-after comparison (ignoring whitespace changes):

```diff
 --- /tmp/before	2022-07-20 21:52:44.336641810 +0930
 +++ /tmp/after	2022-07-20 21:55:54.355487769 +0930
 @@ -1,7 +1,7 @@
 -LIGHTNING-CLI(1)                                          lightning-cli                                         LIGHTNING-CLI(1)
 +LIGHTNING-CLI(1)                                                                                                LIGHTNING-CLI(1)
  
  NAME
 -       lightning-cli - Control lightning daemon
 +       lightning-cli -- Control lightning daemon
  
  SYNOPSIS
         lightning-cli [OPTIONS] command
 @@ -14,10 +14,7 @@
  
          --conf=PATH Sets configuration file (default: lightning-dir/config ).
  
 -        --network=network
 -        --mainnet
 -        --testnet
 -        --signet Sets network explicitly.
 +       --network=network --mainnet --testnet --signet Sets network explicitly.
  
          --rpc-file=FILE Named pipe to use to talk to lightning daemon: default is lightning-rpc in the lightning directory.
  
 @@ -43,8 +40,8 @@
  
          --version/-V Print version number to standard output and exit.
  
 -        allow-deprecated-apis=BOOL  Enable  deprecated options. It defaults to true, but you should set it to false when testing
 -       to ensure that an upgrade won't break your configuration.
 +       allow-deprecated-apis=BOOL Enable deprecated options. It defaults to true, but you should set it to false when testing to
 +       ensure that an upgrade won't break your configuration.
  
  COMMANDS
         lightning-cli simply uses the JSON RPC interface to talk to lightningd, and prints the results. Thus the commands  avail‐
 @@ -67,7 +64,7 @@
  
         lightning-cli help
  
 -              1.     Fund a 10k sat channel using uncomfirmed outputs
 +       2.     Fund a 10k sat channel using uncomfirmed outputs
  
         lightning-cli --keywords fundchannel id=028f...ae7d amount=10000sat minconf=0
  
 @@ -84,4 +81,4 @@
         Note: the modules in the ccan/ directory have their own licenses, but the rest of the code is covered  by  the  BSD-style
         MIT license.
  
 -                                                                                                                LIGHTNING-CLI(1)
 +Core Lightning v0.11.0.1-350-gac2e137                                                                           LIGHTNING-CLI(1)
```

Signed-off-by: Rusty Russell <[email protected]>
Fixes: ElementsProject#5437

doc/Makefile

@@ -125,10 +125,8 @@ RBRACKET=)
 $(MARKDOWN_WITH_SCHEMA): doc/lightning-%.7.md: doc/schemas/%.schema.json tools/fromschema.py
 	@if $(call SHA256STAMP_CHANGED); then $(call VERBOSE, "fromschema $@", tools/fromschema.py --markdownfile=$@ doc/schemas/$*.schema.json > $@.tmp && grep -v SHA256STAMP: $@.tmp > $@ && rm -f $@.tmp && $(call SHA256STAMP,[comment]: # $(LBRACKET),$(RBRACKET))); else touch $@; fi
 
-# mrkd doesn't format nested lists properly, so we fixup with sed (see doc/lightning-connect.7
-# and https://github.com/refi64/mrkd/issues/4
-$(MANPAGES): doc/%: doc/%.md
-	@if $(call SHA256STAMP_CHANGED); then $(call VERBOSE, "mrkd $<", mrkd $< $@.tmp && sed -e 's/\(.\)\.RS$$/\1\n.RS/' < [email protected] > $@ && rm [email protected] && $(call SHA256STAMP,\\\",)); else touch $@; fi
+$(MANPAGES): doc/%: doc/%.md tools/md2man.sh version_gen.h
+	@if $(call SHA256STAMP_CHANGED_ALL); then $(call VERBOSE, "md2man $<", VERSION=$(VERSION) tools/md2man.sh $< > $@ && $(call SHA256STAMP_ALL,\\\",)); else touch $@; fi
 
 $(MANPAGES): $(FORCE)
 $(MARKDOWN_WITH_SCHEMA): $(FORCE)