[RFC] Cleanup user documentation build

This PR does some cleanup in the Makefile of the user documentation.
The first commit makes `make html` work.
Also some generated files got deleted and the `*.1` files are converted to UTF-8.
Instead of nroff, you can use `man` directly:

```diff
diff --git a/runtime/doc/Makefile b/runtime/doc/Makefile
index 34ca935..6098f32 100644
--- a/runtime/doc/Makefile
+++ b/runtime/doc/Makefile
@@ -270,34 +270,32 @@ MANPAGES = \
 	manpages/vimdiff.man \
 	manpages/vimtutor.man \
 	manpages/xxd.man \
-
-# TODO (stefan991): fix output encoding of nroff
-#	manpages/de/vim-de.man \
-#	manpages/fr/vim-fr.man \
-#	manpages/fr/evim-fr.man \
-#	manpages/fr/vimdiff-fr.man \
-#	manpages/fr/vimtutor-fr.man \
-#	manpages/fr/xxd-fr.man \
-#	manpages/pl/vim-pl.man \
-#	manpages/pl/evim-pl.man \
-#	manpages/pl/vimdiff-pl.man \
-#	manpages/pl/vimtutor-pl.man \
-#	manpages/pl/xxd-pl.man \
-#	manpages/it/vim-it.man \
-#	manpages/it/evim-it.man \
-#	manpages/it/vimdiff-it.man \
-#	manpages/it/vimtutor-it.man \
-#	manpages/it/xxd-it.man \
-#	manpages/ru/vim-ru.man \
-#	manpages/ru/evim-ru.man \
-#	manpages/ru/vimdiff-ru.man \
-#	manpages/ru/vimtutor-ru.man \
-#	manpages/ru/xxd-ru.man \
-#	manpages/ja/vim-ja.man \
-#	manpages/ja/evim-ja.man \
-#	manpages/ja/vimdiff-ja.man \
-#	manpages/ja/vimtutor-ja.man \
-#	manpages/ja/xxd-ja.man
+	manpages/de/vim-de.man \
+	manpages/fr/vim-fr.man \
+	manpages/fr/evim-fr.man \
+	manpages/fr/vimdiff-fr.man \
+	manpages/fr/vimtutor-fr.man \
+	manpages/fr/xxd-fr.man \
+	manpages/pl/vim-pl.man \
+	manpages/pl/evim-pl.man \
+	manpages/pl/vimdiff-pl.man \
+	manpages/pl/vimtutor-pl.man \
+	manpages/pl/xxd-pl.man \
+	manpages/it/vim-it.man \
+	manpages/it/evim-it.man \
+	manpages/it/vimdiff-it.man \
+	manpages/it/vimtutor-it.man \
+	manpages/it/xxd-it.man \
+	manpages/ru/vim-ru.man \
+	manpages/ru/evim-ru.man \
+	manpages/ru/vimdiff-ru.man \
+	manpages/ru/vimtutor-ru.man \
+	manpages/ru/xxd-ru.man \
+	manpages/ja/vim-ja.man \
+	manpages/ja/evim-ja.man \
+	manpages/ja/vimdiff-ja.man \
+	manpages/ja/vimtutor-ja.man \
+	manpages/ja/xxd-ja.man
 
 .SUFFIXES:
 .SUFFIXES: .c .o .txt .html .1 .man
@@ -320,7 +318,7 @@ doctags: doctags.c
 manpages: $(MANPAGES)
 
 .1.man:
-	nroff -man $< | sed -e s/.//g > $@
+	man -l -Tutf8 $< | sed -e s/.//g > $@
 
 # Awk version of .txt to .html conversion.
 html: noerrors tags $(HTMLS)
```

This will still show warnings (for `ja`), but the output looks fine:

```
VIM(1)                      General Commands Manual                     VIM(1)

名前
       vim - Vi IMproved, プログラマのテキストエディタ

書式
       vim [options] [file ..]
       vim [options] -

```