[RFC]: Inject copy button during markdown to html conversion

### Description

Add copy button in code blocks in docs.
https://github.com/stdlib-js/www/issues/75

### Related Issues

No

### Questions

No.

### Other

No.

### Checklist

- [x] I have read and understood the [Code of Conduct](https://github.com/stdlib-js/stdlib/blob/develop/CODE_OF_CONDUCT.md).
- [x] Searched for existing issues and pull requests.
- [x] The issue name begins with `RFC:`.

Start by reviewing the issue description and the related stdlib-js/www issue 75 to determine where markdown-to-HTML conversion for documentation occurs. Identify the relevant conversion entry point and existing code-block handling before defining the copy-button behavior. Done means code blocks in the docs render with a working copy button and the conversion behavior is covered by appropriate tests.

索引モデルが issue の本文から書いたものです。