"Usage" page of documentation is outdated

## Describe the bug
When i tried to follow the steps described in the documentation ([usage page](https://flask-webpackext.readthedocs.io/en/latest/usage.html)) i was facing following error while building the assets:

```
> webpack --config webpack.config.js

[webpack-cli] TypeError: Cannot read property 'length' of undefined
at /home/fwoerister/PycharmProjects/webpack/instance/assets/node_modules/webpack-manifest-plugin/lib/plugin.js:129:39
at Array.reduce ()
at ManifestPlugin. (/home/fwoerister/PycharmProjects/webpack/instance/assets/node_modules/webpack-manifest-plugin/lib/plugin.js:116:26)
at Hook.eval [as callAsync] (eval at create (/usr/lib/node_modules/webpack/node_modules/tapable/lib/HookCodeFactory.js:33:10), :9:1)
at Hook.CALL_ASYNC_DELEGATE [as _callAsync] (/usr/lib/node_modules/webpack/node_modules/tapable/lib/Hook.js:18:14)
at Compiler.emitAssets (/usr/lib/node_modules/webpack/lib/Compiler.js:862:19)
at /usr/lib/node_modules/webpack/lib/Compiler.js:450:10
at processTicksAndRejections (internal/process/task_queues.js:77:11)
```

## Steps to Reproduce
Following the steps described on the ["usage" page](https://flask-webpackext.readthedocs.io/en/latest/usage.html) of the documentation.

## Expected behavior
The assets should be built without error

## Additional context
After some investigation I realized that this error was a [known issue](https://github.com/shellscape/webpack-manifest-plugin/pull/224), that was fixed in a more recent version of the plugin. I also found out that in the more recent versions the plugin class was renamed from "ManifestPlugin" to "WebpackManifestPlugin" which has to be fixed in the webpack config as well.

Start with the linked usage page and the webpack config used by its steps. Check the documented webpack-manifest-plugin version and class name against the current plugin behavior; done means the documented process and configuration build the assets without the reported error.

Written by the indexing model from the issue text.