[doc] Completely replace docgen with hugo

This change completely replaces docgen and replaces or removes
docgen-specific markdown in documentation.  It also does the following:

  * Updates all local links to use hugo relative references so that a
    broken link is a broken build.
  * Uses upstream wavedrom, which breaks at least one page that depends
    on local modifications.
  * Renames most hw/ip/**/ip_name.doc and dv_plan documents for a more
    aesthetic document tree layout.
  * Moves some doc/ pages into their own page bundle.
  * Updates util/build_docs.py to pre-generate registers, hwcfg, and
    dashboard fragments and invoke hugo.