`py_wheel`: Summary, Description, Description-Content-Type

# 🚀 feature request and documentation 🐞 bug report

### Relevant Rules

`py_wheel`

### Description

The documentation of the `description_file` attribute of `py_wheel` describes the attribute as "A file containing text describing the package in a single line."

This directly contradicts the core metadata specifications, https://packaging.python.org/en/latest/specifications/core-metadata/#description, which states

> A longer description of the distribution that can run to several paragraphs. Software that deals with metadata should not assume any maximum size for this field, though people shouldn’t include their instruction manual as the description.

There is, however, another field in the core metadata specifications that actually matches the description, https://packaging.python.org/en/latest/specifications/core-metadata/#summary:

> Summary
> A one-line summary of what the distribution does.

I'm not entirely sure if this should be a bug report or a feature request, because both "Description" and "Summary" would be nice to have. I'm assuming that compliance with the [PyPA specifications](https://packaging.python.org/en/latest/specifications/) is intended, in which case it is at least a documentation bug. The `description_file` attribute ends up as the "Description", and can be several paragraphs. But it would be nice to also support the "Summary" field using a different attribute, and it would also be nice to be able to set the "Description-Content-Type field, https://packaging.python.org/en/latest/specifications/core-metadata/#description-content-type, so at least in those parts it's a feature request as well.

### Describe the solution you'd like

1. Documentation of `description_file` updated to support more than one line of description.
2. New attribute `summary` for the one-line description, setting [Summary](https://packaging.python.org/en/latest/specifications/core-metadata/#summary)
3. New attribute `description_content_type` for specifying markup syntax, setting [Description-Content-Type](https://packaging.python.org/en/latest/specifications/core-metadata/#description-content-type)

Start at the `py_wheel` rule and trace how `description_file` is documented and mapped into Python core metadata. Compare that behavior with the linked Summary, Description, and Description-Content-Type specifications, then verify that the documentation and the three requested metadata attributes behave as described.

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