You have less control over formatting and ad placement in the README file, as rendered markdown offers only limited options. With a dedicated documentation website, it’s much easier.
It’s also a question of sovereignty. If your documentation is in the README, then GitHub owns the audience. If they, for some reason, close your project, you’re finished. With your own documentation page, the risk is much lower.
It’s also a question of sovereignty. If your documentation is in the README, then GitHub owns the audience. If they, for some reason, close your project, you’re finished. With your own documentation page, the risk is much lower.