doc: Migrate DesignPrinciples wiki page to Sphinx

Move the current DesignPrinciples wiki page to
doc/develop/designprinciples.rst.  The changes here are for formatting
or slight rewording so that it reads well when linking to other Sphinx
documents.

Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>