Delete docs/Manifest.toml and remove your package from docs/Project.toml.
About the why: you probably created the manifest with julia v1.6, but then you’re building the documentation in GitHub Actions with julia v1.5, but manifests are julia-version-specific and the presence of TOML (a stdlib in v1.6 but not v1.5 confuses the resolver). Having the manifest in CI is a bad idea, unless you consistently use the same version of Julia to generate it and instantiate the environment.
Regarding why removing your package from docs/Project.toml: your package will be dev’ed when building the documentation, but it should not be in the committed Project.toml.