Add CSS class indicating its domain for each desc node

[tk0miya]

Is your feature request related to a problem? Please describe.
As commented at #6990 (comment), it would be better to decorate (change a style) every element of object descriptions. To do that, it would be useful to give a domain-named CSS class for each desc element to select specific domain.

Describe the solution you'd like
Give a domain-named CSS class for each desc element.

<desc classes=['py', 'function']>
  <desc_signature>...</desc_signature>
  <desc_content>...</desc_content>
</desc>

This structure enables themes to give a style with a CSS selector like .py.function > .attribute-name.

Describe alternatives you've considered
N/A

Additional context
N/A