Document the variant filtering feature

[cortinico]

Fixes #3893

This PR adds documentation for the variant filtering feature shipped with the Gradle plugin: getsentry/sentry-android-gradle-plugin#140

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🔍 Inspect: https://vercel.com/sentry/sentry-docs/9DJt11Dh8yAie8ML517uMSU5tRyD
✅ Preview: https://sentry-docs-git-fork-cortinico-nc-gradle-variant-filter.sentry.dev

[imatwawana]

Made minor wording edit, but otherwise looks good to me (pending technical review). Thank you!

[marandaneto]

@cortinico thanks so much :)

[philipphofmann]

Thanks @cortinico 🥇