* docs/*.html, docs/devhelp/*.html, docs/html/*.html: Delete
* docs/.gitignore, docs/devhelp/.gitignore, docs/html/.gitignore:
  Ignore generated docs files
* docs/Makefile.am: Rebuild all HTML docs for 'all' target
  and include them in EXTRA_DIST and CLEANFILES
* docs/devhelp/Makefile.am: Include HTML in CLEANFILES