Add documentation for Web Identity / OIDC configuration

[lukehoban]

We don't have documentation for OIDC configuration via Web Identity. In particular:

• https://www.pulumi.com/registry/packages/aws/installation-configuration/ is missing any mention of these configuration options
• https://www.pulumi.com/registry/packages/aws/installation-configuration/ is also missing any prose helping users to understand this option
• https://www.pulumi.com/registry/packages/aws/api-docs/provider/#providerassumerolewithwebidentity is missing any descriptions (as are many other nested types)

The equivalent upstream content for reference: https://registry.terraform.io/providers/hashicorp/aws/latest/docs#assuming-an-iam-role-using-a-web-identity and https://registry.terraform.io/providers/hashicorp/aws/latest/docs#assume-role-with-web-identity-configuration-reference and https://registry.terraform.io/providers/hashicorp/aws/latest/docs#argument-reference.