Add a note about SOD recipe in SOD introduction

[core-man]

Description of proposed changes

Preview: https://seismo-learn.org/sitepreview/seismo-learn/software/sod/sod/introduction/

Addresses #41

PRs for new software/tutorials

• [x] Software metadata is complete
• [x] The "last updated date" in the metadata page is up-to-date
• [ ] All links work
• [ ] You're listed as the author(s) in the metadata page
• [ ] Anyone who have made significant contributions (e.g., review) to the tutorials are listed in the meteda page
• [ ] Add a test to weekly CI jobs

PRs for bugs/improvements

• [ ] You're listed as the proofreader in the metadata page

[github-actions[bot]]

This comment was written by the Continuous Documentation bot!

• Preview: https://seismo-learn.org/sitepreview/seismo-learn/software/sod/index.html
• Commit hash: 6582b2006f1b9eff4e96f8039ba97ea828915476

[core-man]

@seisman I may begin to work on SOD tutorials on software and SODrecipes by a series of PRs.