Restructure the existing documentation to group the content by intended
audience. This produces four sections:
* An introduction section, relevant to "newcomers" to Ibex.
* An user guide, intended for hardware designers (integrators) and
software developers who want to integrate Ibex, and develop software
for it.
* A reference guide, which provides background information on the
design. This section is essential when working on Ibex, and also
documents our design decisions.
* A developer guide aimed at people modifying Ibex itself. It consists
mostly of process and tool documentation: how to run the verification
after a code change, how to use GitHub, etc.
This commit is large, but text is mostly unchanged. A couple of
introductions and tables of content were added, but no significant
changes to the text have been made. These will be done in follow-ups.
Signed-off-by: Philipp Wagner <phw@lowrisc.org>
