Partial fix for PDF doc build

[pushfoo]

TL;DR: replaced 403ing images + added gif converter, but LaTex still broken

Changes

• Replaced missing (403) links starting with camo.githubusercontent
  • When screenshots were available, I used those
  • For when they weren't, there's a pngcrushed placeholder image
• Added imageconvert extension for PDF to convert gifs
• Cleaned up Boxhead Survivor sampel game links + made them clickable
• Added LaTeX build requirements notes

What's Still Broken / Untested

PDF Build: Still Broken

Running ./make.py latexpdf produces plenty of warnings and ends with this error and a LaTeX input prompt:

LaTeX Warning: Hyper reference `tutorials/platform_tutorial/index:platformer-tu
torial' on page 67 undefined on input line 2403.
[67 <./YI7uLX.jpg>] [68 <./screenshot1.png>]
Runaway argument?
{{example
! Paragraph ended before \\sphinxsafeincludegraphics was complete.
<to be read again> \par l.2483 ? 

EPUB build: Untested

Haven't gotten to this yet.

[pushfoo]

The test failures appear to be pyright upset about the GUI.

[pushfoo]

Rebased onto development to include #2062.