[otbn/doc] Expand programming sections
[otbn/doc] Expand programming sections

Split the "Programming" sections into two parts: a section about writing
software running on OTBN itself, and a section about writing software
running on the host CPU (Ibex). In particular, I made the following
changes:

* Move the general information about IMEM and DMEM to the "Design
  Details" sections.
* Expand the "High-level operation sequence" section.
* Add a "Driver" section, mentioning the existence of the newly added
  "high-level" driver.
* Add a section "Writing OTBN applications" with new subsections on
  toolchain support, data passing, and returning from an OTBN
  application.

Fixes #2804

Signed-off-by: Philipp Wagner <5a4b10454c30419e54a1570be358a516ff84bb63@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan