[FIX] doc: various

* fix Sphinx 1.6 compatibility leading to the "tiles" on the home page
  not working anymore: 1.6 replaces BuildEnvironment.reolve_toctree by
  TocTree().resolve(), while the method still exists it's not actually
  called anymore
* rejigger some CSS as the second section went from one big tile to 3
  smaller tiles, and got laid out as a row rather than a second 2x2
  block. Recode the entire mess with flexbox, remove some stuff which
  conflicted with boostrap (this screen should probably be
  de-boostrapped and completely converted to flexbox or grid)
* add building CSS from LESS to the makefile, all Odoo devs should
  have less installed locally (for assets)