Add tutorial for downloading, reading, writing and plotting catalog

[seisman]

Closes https://github.com/seismo-learn/seismology101/issues/549.

[github-actions[bot]]

This comment was written by the Continuous Documentation bot!

• Preview: https://seismo-learn.org/sitepreview/seismo-learn/seismology101/obspy-get-events/index.html
• Commit hash: 8914cd9c68af94d6dcb0722541d9fb2b50b24a01

[core-man]

Looks great~