[426583] Use PNG with no shadows for the architecture diagrams

The SVG used before can be badly rendered depending on the embedded
browser.

Bug: 426583
Change-Id: Iadb09fa2e40459f99f45bd0e9e05fd0ad47c3885
Signed-off-by: Pierre-Charles David <pierre-charles.david@obeo.fr>
[426583] Fix the screenshots in the "custom style" extension point doc

Bug: 426583
Change-Id: If68ea52c245ab57cd2bed14d4f31749b3b53b698
Signed-off-by: Florian Barbin <florian.barbin@obeo.fr>
[426583] Fix the screenshots in the "tab-bar extensions" extension point
doc

Bug: 426583
Change-Id: I64479faca1c8d9d8ea650d58de55db5709ca3b3d
Signed-off-by: Florian Barbin <florian.barbin@obeo.fr>