sod: Add introduction to recipe

[core-man]

Description of proposed changes

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

References:

• https://github.com/seisman/seisman.info.posts/edit/master/post/2016-12-24-sod-notes.md
• https://blog.seisman.info/sod-notes/#sod-%E7%9A%84%E4%BD%BF%E7%94%A8
• https://blog.seisman.info/sod-notes/#xml-%E6%A0%BC%E5%BC%8F%E7%AE%80%E4%BB%8B
• https://blog.seisman.info/sod-notes/#sod-recipe-%E7%BB%93%E6%9E%84

Addresses #41

PRs for new software/tutorials

• [ ] Software metadata is complete
• [ ] 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

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• Preview: https://seismo-learn.org/sitepreview/seismo-learn/software/sod-recipe/index.html
• Commit hash: e49fd10162040ba24493b25c73d7c4dcd16f6a55