Add short summary field to dataset and clean up repr

[BrendanSchell]

Description

Updates Dataset with summary field and revises its standalone and collection repr with this and some additional changes to appear cleaner.

Updated Dataset

• Adds a summary field to Dataset and CreateDataset that contains a shorter text description of the dataset.
• The html repr is updated to show this summary along with the full dataset description (converted from markdown to html).
• Updated header levels in dataset repr (now h1 and h2 so that description markdown can be level 3 and below while still being child of Description header)
• Added some additional h2 headers to section off the different pieces of the repr
  

Updated dataset collection

• Summary field appears as part of the dataset fields when listed as a collection.
  

Styling changes

• Changed some text styling to use variable for syft-dataset fonts so so that it's legible in VS code notebooks in dark mode.

Example usage for adding summary

dataset = sy.Dataset(
    name="my dataset 3",
    asset_list=[
        sy.Asset(name="my asset", data=np.array([1, 2, 3]), mock=np.array([1, 1, 1]))
    ],
    summary="This is a short summary of the dataset.",
    description="Long description",
)

Closes https://github.com/OpenMined/Heartbeat/issues/1281.

Affected Dependencies

List any dependencies that are required for this change.

How has this been tested?

• Tested manually to make sure it displays ok
• Tested migrations manually using dataset.migrate_to(version) for both CreateDataset and Dataset

Checklist

• I have followed the Contribution Guidelines and Code of Conduct
• I have commented my code following the OpenMined Styleguide
• I have labeled this PR with the relevant Type labels
• My changes are covered by tests

[tcp]

@BrendanSchell if you wanna pair so we take care of the repr in different modes, let me know :)

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