Replace with asset pipeline

### Describe the bug

With netlify migration, now we use middleman as framework.
In the previous sinatra application (Heroku), we used customized version of
sinatra-asset-pipeline.

Now for migration simplicity, we don't use asset pipeline yet.

### Link to the problematic contents

It still use non-compressed, separated js or css.

* source/layouts/layout.erb
* source/layotus/minimal_layout.erb

### Expected explanation

Introduce asset pipeline and restructure assets directory.

### Additional context

_No response_