add gif support

[m-reuter]

Add GIF Image Support to nbsphinx

Summary

This pull request adds support for rendering animated GIF images in Jupyter notebooks when using nbsphinx (see also #864 ). The implementation follows the same approach used in similar projects like WhipperSnapPy and the nbconvert PR for GIF support.
Note that we do not add a monkey patch for nbconvert here, so first the PR upstream needs to be merged before this will work.

Changes Made

1. Added image/gif to Display Data Priority (HTML)

• File: src/nbsphinx/__init__.py
• Change: Added 'image/gif' to the DISPLAY_DATA_PRIORITY_HTML tuple
• Purpose: Ensures GIF images are considered as valid output types when rendering HTML output, with appropriate priority relative to other image formats

2. Added GIF File Extension Mapping

• File: src/nbsphinx/__init__.py
• Change: Added 'image/gif': '.gif' to the THUMBNAIL_MIME_TYPES dictionary
• Purpose: Allows the system to correctly identify and use .gif file extensions when saving GIF images as thumbnails or standalone files

3. Updated RST Template for Image Directive

• File: src/nbsphinx/__init__.py (RST_TEMPLATE)
• Change: Added 'image/gif' to the image directive handling condition in the Jinja2 template
• Purpose: Enables proper rendering of GIF images using reStructuredText's image directive