Cleanup of get-started

[ferafiks]

Description

This is a general cleanup for the get-started section. A lot of unused images were removed and moved, as a lot of other sections (mainly game-studio) relied on images in this section.

Building with docfx produces zero warnings and I checked thoroughly for any unlinked references by searching through all files in zed. On some pages (mainly those containing a video player) the warnings wouldn't show up, as docfx doesn't check html code.

This PR also makes get-started media fully webp.

Changes

• Removes unused images in get-started.
• Moves images that were used in other sections, but not in get-started.
• Rewords the text about project structure in key-contepts, to include the new terminology (project packages). In the future once Rewrite of the files and folders section #491 gets merged, there should be links to the corresponding pages.
• Adds missing redirect urls to the pages that were removed / moved.

Issues

• Partially addresses chore: Remove all Xenko / unlinked images #349