Automatically run oxipng on example screenshots

[Chubercik]

• Fixes Automatically run pngcrush on example screenshots #1534.
• This PR adds an image compression step to the Update Screenshots GitHub Action.
• The whole compression overhead (installing oxipng and running it with the maximum compression level) adds about a minute to the workflow runtime, which is minuscule when compared to the avg. ~3h completion time.

[Chubercik]

By comparing the artifact sizes of the last official GA workflow run and the one run on this PR, we can see noticeable reductions:

Artifact	Current	This PR	Size reduction
screenshots-0	4.5 MB	3.47 MB	-22.9%
screenshots-1	7.05 MB	5.69 MB	-19.3%
screenshots-2	976 KB	565 KB	-42.1%
screenshots-3	3.34 MB	1.98 MB	-40.7%
screenshots-4	1.28 MB	749 KB	-41.5%
screenshots-5	3 MB	1.93 MB	-35.7%
screenshots-6	3.1 MB	1.99 MB	-35.8%
screenshots-7	3.28 MB	2.26 MB	-31.1%
screenshots-8	3.63 MB	2.57 MB	-29.2%
screenshots-9	4.61 MB	2.66 MB	-42.3%
screenshots-10	1.77 MB	1.21 MB	-32.7%
screenshots-11	3.87 MB	3.06 MB	-20.9%
screenshots-12	1.95 MB	1.53 MB	-21.5%
all screenshots	42.36 MB	29.66 MB	-30%

It's worth noting that the sizes from the current workflow aren't the same as image sizes that would be used on the website, given the fact that @mockersf used pngcrush on them manually before merging #1530; still, we can see that this PR achieves better compression when comparing actual size changes in the GH "Files changed" tabs:

• current: https://github.com/bevyengine/bevy-website/pull/1530/files
• this PR: https://github.com/Chubercik/bevy-website/pull/1/files

A few examples:

Image	Preview	Current	This PR
2D Rendering/2d_shapes.png		-9.81 KB (53%)	-14.4 KB (31%)
2D Rendering/2d_viewport_to_world.png		-11.2 KB (25%)	-14.7 KB (1.4%)
2D Rendering/mesh2d_arcs.png		+18.3 KB	+10.6 KB
Window/screenshot.png		-131 KB (58%)	-177 KB (44%)
ECS (Entity Component System)/observers.png		+232 KB	+210 KB

[TrialDragon]

LGTM.

[janhohenheim]

LGTM, just left a note for optimizing speed

[Chubercik]

With @janhohenheim's suggestion applied, oxipng's install time has been brought down to ~1 second, effectively cutting the total compression overhead to around half its previous length ^^

[mockersf]

Could you remove cargo-install usage?

[BD103]

Looks good, thanks for doing this!

I have two notes for how to improve the workflow, but they're nits that don't prevent this from being merged. :)

[BD103]

@alice-i-cecile would you mind merging this, when you have the chance? I think it was missed in today's merge train. (Which is understandable, given how many PRs you had to merge today!)