[depends on #280] Add method for rendering model cards

[codesue]

What does this pull request do?

Adds a method for rendering model cards. This allows you to render a model card by calling model_card.render() without first having to first create a ModelCardToolkit object and scaffold assets (which copies default template files and creates a model card proto). Additionally, saving the rendered model card document to a file is optional with this method.

Current usage:

import model_card_toolkit as mct

toolkit = mct.ModelCardToolkit()
model_card = toolkit.scaffold_assets()
model_card.model_details.name = 'My Model'
model_card_html = toolkit.export_format()

With this change:

import model_card_toolkit as mct

model_card = mct.ModelCard()
model_card.model_details.name = 'My Model'
model_card_html = model_card.render()

Relates to https://github.com/tensorflow/model-card-toolkit/discussions/276.

How did you test this change?

Added unit tests and ran pytest model_card_toolkit. Example notebook: https://colab.research.google.com/gist/codesue/e315e2d79d564ed60a454b8509ca3c31/dev.ipynb

How did you document this change?

Docstrings

Before submitting

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• [x] Read the How to Contribute guide if this is your first contribution.
• [x] Open an issue or discussion topic to discuss this change.
• [x] Write new tests if applicable.
• [x] Update documentation if applicable.