Add transparency information to social preview documentation

[aisgbnok]

Code of Conduct

• I have read and agree to the GitHub Docs project's Code of Conduct

What article on docs.github.com is affected?

Customizing your repository's social media preview

What part(s) of the article would you like to see updated?

Github social preview supports using transparent PNGs, but this does not seem to be documented. Adding a small subsection about transparency would be welcome. Usage of transparent PNGs can be helpful for branding as many platforms support dark/light themes and transparent PNGs can work well with multiple themes.

Additional information

You can read my general GitHub feedback here community/community#13109 (comment), for more information.

[welcome]

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[devines417]