[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.
Seems that OS X man doesn't support the -l and -T option:
```
stefan at air in ~/Dev/neovim/runtime/doc on git:cleanup-docs+
% make manpages
man -l -Tutf8 manpages/vim.1 | sed -e s/.//g > manpages/vim.man
man: invalid option -- l
man, version 1.6c

usage: man [-adfhktwW] [section] [-M path] [-P pager] [-S list]
	[-m system] [-p string] name ...

  a : find all matching entries
  c : do not use cat file
  d : print gobs of debugging information
  D : as for -d, but also display the pages
  f : same as whatis(1)
  h : print this help message
  k : same as apropos(1)
  K : search for a string in all pages
  t : use troff to format pages for printing
  w : print location of man page(s) that would be displayed
      (if no name given: print directories that would be searched)
  W : as for -w, but display filenames only

  C file   : use `file' as configuration file
  M path   : set search path for manual pages to `path'
  P pager  : use program `pager' to display pages
  S list   : colon separated section list
  m system : search for alternate system's man pages
  p string : string tells which preprocessors to run
               e - [n]eqn(1)   p - pic(1)    t - tbl(1)
               g - grap(1)     r - refer(1)  v - vgrind(1)
man -l -Tutf8 manpages/evim.1 | sed -e s/.//g > manpages/evim.man
man: invalid option -- l
man, version 1.6c
...
```
If I start man without options, the encoding is still wrong.
Using this I get files with empty lines:
```
.1.man:
  man $< | sed -e s/.//g > $@
```