move CSS build to webpack (#9983)

- added new 'make webpack' target
- deprecated 'make js' and 'make css'
- extend webpack config to load the less files
- updated docs

I had to rename the source file of `arc-green.less` to avoid generating
a useless JS entrypoint via webpack-fix-style-only-entries which would
not work with different source/destination filenames. I hear that there
should be cleaner solutions possible once we upgrade to Webpack 5.

Co-authored-by: zeripath <art27@cantab.net>

doc/advanced/hacking-on-gitea.en-us.md

@@ -140,24 +140,21 @@ You should run revive, vet and spell-check on the code with:
 make revive vet misspell-check
 ```
 
-### Working on CSS
+### Working on JS and CSS
 
-Edit files in `web_src/less` and run the linter and build the CSS files via:
+Edit files in `web_src` and run the linter and build the files in `public`:
 
 ```bash
-make css
+make webpack
-```
-
-### Working on JS
-
-Edit files in `web_src/js`, run the linter and build the JS files via:
-
-```bash
-make js
 ```
 
 Note: When working on frontend code, it is advisable to set `USE_SERVICE_WORKER` to `false` in `app.ini` which will prevent undesirable caching of frontend assets.
 
+### Building Images
+
+To build the images, ImageMagick, `inkscape` and `zopflipng` binaries must be available in
+your `PATH` to run `make generate-images`.
+
 ### Updating the API
 
 When creating new API routes or modifying existing API routes, you **MUST**