Use SVG images instead of PNG

[peacekeeper]

Some images are currently in PNG format, we should replace those with SVG.

Also, some images show mostly (example) text with some coloring or other formatting, those should be replaced with HTML+CSS.

[peacekeeper]

Update on this issue is that most images are now in SVG format, except for a few that mostly consist of text and should be replaced with HTML+CSS, in the following sections:

• https://www.w3.org/TR/did-resolution/#did-resolution-examples
• https://www.w3.org/TR/did-resolution/#did-url-dereferencing-examples

[peacekeeper]

All PNG images have now been either replaced by SVG or by regular text.