[Feature] Document difference between echarts and echarts.simple

### What problem does this feature solve?

New user here. I use chartjs which is about 250KiB, whereas echarts (`dist/echarts.min.js`) is about 1MiB. Massive difference!

So I was surprised to find another file `dist/echarts.simple.min.js` which is 450KiB. Still large, but a significantly better option. (There is also a "common" file.)

However, it is not documented anywhere. I have no way to know what it is or how it differs from the main js file.

### What does the proposed API look like?

Please add the difference between these two files to the installation page, so users can make an informed decision about which to use.

Thanks!