Feature Announcement: Relative X-min

[dreeves]

### Desiderata
- [x] Have someone read a draft
- [x] Keywords/tags
- [x] Title image
- [x] Link preview excerpt
- [x] Publish at https://blog.beeminder.com/relativity on 2024-08-21
- [x] Blog blurbs
- [x] Waggledance
- [x] Tweet at https://twitter.com/bmndr/status/1826671086215594458 on 2024-08-22
- [ ] Tip of the day

https://doc.bmndr.co/relativity

Verbata: feature announcements, relative tmin, relative t-min,

[bsoule]

[dreeves]