document how to use custom webpack configuration with workers sites

[ashleymichal]

as of cloudflare/wrangler-legacy#957, Wrangler supports use of custom webpack configuration for Workers Sites projects. However, because of the standard directory structure of a Site project, it's not abundantly obvious that you might need to be careful about how you configure your entry. We should document this in the webpack section of the docs, and link to it from the Workers Sites section.