feat(v2): configureWebpack merge strategy + use file-loader for common asset types (#2994)

* Add some default asset loaders
Add webpack merge strategy feature to enable plugins to prepend some webpack configuration (like the ideal image plugin that should override the default image loader)

* Add documentation for using assets from markdown

* add path prefix for webpack file loader

* renaming

* document Merge strategies

* rename mergeStrategies -> mergeStrategy

packages/docusaurus-plugin-ideal-image/src/index.ts

@@ -25,6 +25,9 @@ export default function (
 
     configureWebpack(_config: Configuration, isServer: boolean) {
       return {
+        mergeStrategy: {
+          'module.rules': 'prepend',
+        },
         module: {
           rules: [
             {