Second attempt to get Hydra to see style.css & co.

Dependencies

• [2] U6MQV7QL Specify all of $(htmldir) as the doc.
• [3] Z45QO5BP hydra/release.nix: posthook -> postdist
• [4] VXNVNOK5 release.nix: Install the manual under the right prefix.
• [5] JXLJS4RI release.nix: Make the HTML manual a Hydra build product.
• [*] T4LLYESZ * Nix expression for building Hydra.

Change contents

• replacement in release.nix at line 36
  [3.88]→[2.0:52](∅→∅)
  − echo "doc manual $out/share/doc/hydra" >> \
  [3.88]

  [3.152]
  + echo "doc manual $out/share/doc/hydra manual.html" >> \