PEP 817: Final set of pre-split updates

This is a set of updates to PEP 817 that were prepared prior to the
decision to split it into smaller PEPs.  We do not intend to pursue
PEP 817 in this updated form.  However, we would like to get the changes
committed to the repository so that the history of changes is not lost.

The changes include refocusing of the security details, some
clarifications, tool behavior outlines and update for the images to
respect dark theme.

We are not marking the PEP as withdrawn, as our current plan is to
eventually replace the specification part with an index listing the
split PEPs.

Signed-off-by: Michał Górny <mgorny@quansight.com>

Author: mgorny