[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.
Building with the following lines uncommented works for me, although there are some warnings for japanese, e.g.
> manpages/ja/xxd-ja.1:84: warning [p 1, 8.5i]: can't break line
manpages/ja/xxd-ja.1:104: warning [p 2, 0.3i]: cannot adjust line
manpages/ja/vim-ja.1:426: warning [p 5, 9.8i]: cannot adjust line
manpages/ja/xxd-ja.1:134: warning [p 2, 4.0i]: cannot adjust line
manpages/ja/xxd-ja.1:343: warning [p 4, 3.5i]: cannot adjust line

The output is UTF-8, though:

```
$ file manpages/ja/vim-ja.man
manpages/ja/vim-ja.man: UTF-8 Unicode text
```

Am I missing something?