Url Checker

[antoinebou12]

This pull request includes various updates to documentation files and the addition of a GitHub Actions workflow for URL checking. The most important changes include adding a URL checking workflow, updating badges and links in documentation files, and correcting references in tutorials.

Workflow Addition:

• Added a GitHub Actions workflow to check URLs in documentation files. This workflow runs on push, pull_request, and workflow_dispatch events. (.github/workflows/url-check.yml)

Documentation Updates:

• Updated badges in docs/index.md and docs/python.md to include version, latest release date, platforms, and license. (docs/index.md, docs/python.md) [1] [2]
• Corrected links in docs/polyfempy_doc.md to point to the appropriate sections on the PolyFEM website. (docs/polyfempy_doc.md) [1] [2] [3] [4] [5] [6] [7]

Tutorial Updates:

• Updated the GitHub repository links in the sling-shot and sphere-pushing-box tutorials to point to the correct repositories. (docs/tutorials/sling-shot/sling-shot.md, docs/tutorials/sphere-pushing-box/sphere-pushing-box.md) [1] [2]
• Updated the file reference in the trajectory-optimization tutorial to point to the correct JSON file. (docs/tutorials/trajectory-optimization/trajectory-optimization.md)