Update nbsphinx to Recognize text/x-rst Mimetype for Raw Cells #833
Description
With the release of [nbconvert version 7.16.5](https://github.com/jupyter/nbconvert/blob/main/CHANGELOG.md#7165 according to nbconvert) on 2025-01-02, the mimetype for reStructuredText raw cells has been updated from text/restructuredtext to text/x-rst . This change affects nbsphinx’s ability to detect titles in notebooks where the first cell is a raw reStructuredText cell. 
Current Behavior:
When the first cell of a notebook is a raw reStructuredText cell with the mimetype text/x-rst, nbsphinx does not recognize the document’s title, leading to warnings such as:
WARNING: toctree contains reference to document 'path/to/notebook' that doesn't have a title: no link will be generated
Proposed Solution:
Modify nbsphinx to recognize both text/restructuredtext and text/x-rst mimetypes when processing raw cells. Specifically, update the condition in the nbsphinx source code to:
{%- elif raw_mimetype in ['text/restructuredtext', 'text/x-rst'] %}This change will ensure compatibility with notebooks using either mimetype.
This issue will be fixed by PR #834