doc: Re-add Docbook source files to the distribution.

Dependencies

• [2] PD4VHG3K doc: Use a CSS and callout images for the HTML output.
• [3] FVMVRIRR doc: Ship Docbook's callout images.
• [4] PKE6I67S doc: Import the "Creating Projects" chapter by Visser & Dolstra.
• [5] FV2M6MOT hydra: use autoconf/-make
• [6] 6U6KBFYP Build the HTML manual; add the HTML & PDF to the distribution; install them.

Change contents

• replacement in doc/manual/Makefile.am at line 3
  [4.17009]→[4.17009:17050](∅→∅)
  − EXTRA_DIST = $(DOCBOOK_FILES) manual.pdf
  [4.17009]

  [2.0]
  + EXTRA_DIST = $(DOCBOOK_FILES)
• replacement in doc/manual/Makefile.am at line 19
  [3.54]→[3.54:74](∅→∅)
  − EXTRA_DIST = images
  [3.54]

  [3.74]
  + EXTRA_DIST += images