manual: Add index.html

Dependencies

• [2] PD4VHG3K doc: Use a CSS and callout images for the HTML output.
• [*] FV2M6MOT hydra: use autoconf/-make

Change contents

• edit in doc/manual/Makefile.am at line 31
  [2.629]

  [2.629]
  + ln -sfn manual.html $(DESTDIR)$(htmldir)/index.html