Revise Safety Impact `reference` page

[sei-renae]

The Safety Impact page at https://certcc.github.io/SSVC/reference/decision_points/safety_impact should be revised for the following:

• The Gathering Info section is dissimilar from other Gathering Info pages and is more similar to "Considerations about Safety Impact".
• The page is altogether out of sync with our shift toward IEC 61508 framing and away from FAA rules.
• Unlike other pages, the tone is more academic than casual.