docs: Significantly reduce size of image struggling to load

[xTVaser]

The background image was taking way too long to load, changed it to a jpg

[coveralls]

Pull Request Test Coverage Report for Build 891682242

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.004%) to 69.773%

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Files with Coverage Reduction	New Missed Lines	%
game/system/iop_thread.cpp	1	96.08%

Totals
Change from base Build 891517393:	0.004%
Covered Lines:	35982
Relevant Lines:	51570

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💛 - Coveralls

[ManDude]

You can also try something like PNGGauntlet to squeeze PNGs. Tried it on the original background and it went from 6600KB to 1798KB.