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>

web_src/less/themes/_base.less

@@ -1,3 +0,0 @@
-// TODO: Instead of having each theme file define each
-//       CSS/LESS item in this file and then overide
-//       in the theme files

web_src/less/themes/theme-arc-green.less

@@ -1,5 +1,3 @@
-@import "_base";
-
 .hljs {
     display: block;
     overflow-x: auto;