docs(configuration): add entry[x].runtime

[hai-x]

Add entry[x].runtime configuration that also align with https://webpack.js.org/concepts/entry-points/#entrydescription-object

Related pr
webpack/webpack#11235

• Read and sign the [CLA][1]. PRs that haven't signed it won't be accepted.
• Make sure your PR complies with the [writer's guide][2].
• Review the diff carefully as sometimes this can reveal issues.
• Do not abandon your Pull Request: [Stale Pull Requests][3].

[vercel]

@hai-x is attempting to deploy a commit to the Webpack Docs Team on Vercel.

A member of the Team first needs to authorize it.