Add dev-docs for webpack.

Signed-off-by: David Mehren <dmehren1@gmail.com>

docs/dev/webpack.md

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+# Webpack Docs
+## `webpack.common.js`
+This file contains all common definition for chunks and plugins, that are needed by the whole app.
+
+**TODO:** Document which entry points are used for what.
+
+## `webpack.htmlexport.js`
+Separate config for the "save as html" feature.
+Packs all CSS from `public/js/htmlExport.js` to `build/html.min.css`.
+This file is then downloaded by client-side JS and used to create the HTML.
+See `exportToHTML()` in `public/js/extra.js`.
+
+
+## `webpack.dev.js`
+The development config uses both common configs, enables development mode and enables "cheap" source maps (lines only).
+If you need more detailed source maps while developing, you might want to use the `source-maps` option.
+See https://webpack.js.org/configuration/devtool/ for details.
+
+## `webpack.prod.js`
+The production config uses both common configs and enables production mode.
+This automatically enables various optimizations (e.g. UglifyJS). See https://webpack.js.org/concepts/mode/ for details.
+
+For the global app config, the name of the emitted chunks is changed to include the content hash.
+See https://webpack.js.org/guides/caching/ on why this is a good idea.
+
+For the HTML export config, CSS minification is enabled.