This pull request introduces the ability to add descriptions to tags within the blog application, enhancing the tag functionality by allowing for more detailed information about each tag to be stored and displayed.

• Adds a description TextField to the Tag model in blog/models.py, allowing descriptions to be stored for each tag.
• Updates the TagAdmin class in blog/admin.py to include the description field, enabling it to be edited through the Django admin interface.
• Modifies the view handling /tags/x/ in blog/views.py to include the tag's description in the context passed to the template, making the description available for display.
• Updates the template templates/archive_tag.html to display the tag's description at the top of the page if it exists, providing users with more context about the tag.
• Adds a migration file blog/migrations/0020_tag_description.py to apply the changes to the database schema, ensuring the new description field is properly integrated.

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