PR: Optimize frequently-loaded & loading critical tour images for Spyder 5

[CAM-Gerlach]

Pull Request

Pull Request Checklist

• [x] Read and followed this repo's Contributing Guidelines
• [x] Based your PR on the latest version of the correct branch (master or 4.x)
• [x] Checked your writing carefully for correct English spelling, grammar, etc
• [x] Described your changes and the motivation for them below

Description of Changes

Followup to PR #294 for the tour on Spyder 5. Also includes optimizing the frequently hit logo and readme screenshot (GH homepage image).

As on PR #294, load time is critical for the tour images since it results in significant perceptible delay for the user, blocks the various scripts from firing and in some scenarios, can even cause unpredictable bugs. Running optipng on the images reduced their size by a consistent 30-40% for free, with zero other impacts, so it makes sense to do this here (for the many other unoptimized images, repo space is more critical than load time, and if they were merged unoptimized, we will never be able to recover it and in fact optimizing will only increase the repo size, so I haven't touched them).

[CAM-Gerlach]

As on PR #294 , since this PR is trivial, purely technical and makes no changes to anything user-visible or does anything but standard image optimization, and there is already a large backlog of PRs waiting on the other maintainers to review (which will only get longer due to me being banned from reviewing non-core-dev PRs), I'll go ahead and merge this once the checks pass and I double-check the preview looks identical to before.