Docs: Explain build.default.ini in main docs homepage

[g105b]

James was caught out by this one: https://github.com/orgs/phpgt/discussions/725#discussioncomment-17627788

[g105b]

To add to this, the build.ini example within the build docs uses a different entry point script to what the WebEngine default uses. This should be documented better.

On top of these tweaks, a big link to the WebEngine/ClientSideCompilation docs should be at the top of the page, and on that page there should be an explanation of all of the config for sass, es65, etc.