doc: add new :dtcompatible: role / directive

This allows RST documentation to do something like this:

   See :dtcompatible:`vnd,foo` for more information.

This is transformed into a link to documentation for the "vnd,foo"
compatible.

Some devicetree compatibles may be handled by multiple bindings.
This can happen when the binding file is bus-dependent. Therefore, to
make this work, we need to change the way gen_devicetree_rest.py works
to ensure we have a good source of information for this compatible
regardless of how many bindings are associated with as follows:

- When only a single binding is associated with a compatible, the
  :dtcompatible: link goes directly to the per-binding HTML page.

- When multiple bindings are associated with a compatible, this goes
  to a new generated "disambiguation" page which links to all the
  per-binding pages.

To avoid clashes, we stick the disambiguation pages into a
compatibles/ subdirectory of the generated bindings index root
directory.

Also reorganize the generated bindings output directory into
.../bindings/subdir/binding.rst files. For example,
dts/bindings/arm/arm,dtcm.yaml now gets its generated content in
.../bindings/arm/arm,dtcm.rst.

This brings the 'category' of binding (like 'sensor', 'i2c', etc.)
into the URL, which is a useful hint.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>