chore: reduce size of assets/images

[hoxbro]

These are actually part of the build PyPI + Conda, so reducing the size of them is a good thing

Have used ImageOptim:

Only took the images larger than 1MB.

[codspeed-hq]

Merging this PR will degrade performance by 16.97%

⚡ 1 improved benchmark
❌ 1 regressed benchmark
✅ 50 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	test_quadmesh_curvilinear[256]	54.5 ms	48.5 ms	+12.44%
❌	Simulation	test_layout[forceatlas2_layout]	50.4 ms	60.7 ms	-16.97%

---

_{Comparing reduce_assets (9382659) with main (c18c685)}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.38%. Comparing base (b1bfa3b) to head (9382659).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1479      +/-   ##
==========================================
- Coverage   88.43%   88.38%   -0.06%     
==========================================
  Files          96       97       +1     
  Lines       19183    19432     +249     
==========================================
+ Hits        16965    17175     +210     
- Misses       2218     2257      +39     

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