chore: Update logo for dark or light theme

[mattjohnsonpint]

GitHub allow you to set a theme, which come in both dark and light flavors.

Sentry's black logo looks horrible on a dark theme, and "pixels matter". This is the current state, on a dark theme:

This PR changes the logo to use the purple Sentry logo, which is readable on both white and black backgrounds.

Note: The first draft of this PR used a media query to select either the white logo for a dark theme, or a dark logo for a light theme. However, it was pointed out to me that some repos render their readme files on package manager sites, and most of those don't change their background correctly for OS theme. Thus switching to a white logo when the OS was set to dark mode would give a white-on-white invisible logo. Since this readme is published to PyPi here, it makes sense to use the purple logo.

[mattjohnsonpint]

Some discussion on this getsentry/sentry-electron#479 - apparently this approach doesn't work best due to how other package registries render the readme. Going to try another approach.

[mattjohnsonpint]

This repo publishes to PyPi, so we need to use the purple logo instead. See getsentry/sentry-electron#479 (comment) for more details. Thanks.

[mwarkentin]

Went down a bit of a rabbit hole on this one previously: #33799

[taylangocmen]

Thanks

[mwarkentin]

@mattjohnsonpint Looks like there's a new way to support both light/dark mode now: https://github.blog/changelog/2022-05-19-specify-theme-context-for-images-in-markdown-beta/

Not sure if that would be handled better by downstream repositories or not.

[mattjohnsonpint]

@mwarkentin - That was the approach used in my first draft. It works great when the readme is rendered on GitHub. Unfortunately, most readme files also make their way to package managers and software repositories (PyPi, NPM, Packagist, NuGet, etc.) and very few of those sites render their own background according to the user's theme preferences. For example, this project's readme on PyPi) is always rendered on a white background, regardless of theme settings. If you happen to set your OS for dark theme, then the Sentry logo would render as white by picking up the theme preference, and it would be invisible on the white background.

Hence, why I updated this PR to just use the purple logo.