📖 Document dark mode support for amp-img via media attribute - #40550

All PRs need approval from at least one member of the reviewer set: ampproject/reviewers-amphtml [twifkak, gregable, amaltas, cramforce, jridgewell, westonruter, alanorozco, fikus, erwinmombay, sebastianbenz, patrickkettner, mmmicedcoffee, powerivq, honeybadgerdontcare, chenshay, gmajoulet, keithwrightbos, danielrozenberg, coreymasanto, jonkeller, zikas, processprocess, banaag, enriqe, dethstrobe, kmh287, nainar, caroqliu, raxsha, calebcordry, glevitzky, dreamofabear, mszylkowski, crystalonscript, mrjoro, krdwan, morsssss, alin04, estherkim, rbeckthomas, rileyajones], renovate-approve[bot]

Current Coverage

• src/builtins/amp-img/amp-img.md (raza2004)
• test/manual/amp-img-dark-mode.amp.html (raza2004)
• test/unit/test-custom-element.js (raza2004)

For a description of the OWNERS file syntax, see this example file.
To see the current ownership hierarchy and groups, see the full OWNERS tree.