Add hex sticker

## What

Add a hex sticker for greta.gp.

The sticker is designed and rendered. It comes from `greta.hexes`, which holds
the whole family — that repo is not pushed yet, so the file is currently local:

```
png/light/stacked/greta-gp.png
```

600 dpi, 43.9 × 50.8 mm, transparent outside the hexagon.

## The symbol

A **fan of draws from a GP posterior**, conditioned on five points with an RBF
kernel — `rbf()`. A GP *is* a distribution over functions, so a bundle of
functions is the most literal picture of one; the alternative was a mean with a
credible band, which is what *any* Bayesian regression looks like.

The draws are many, fine and faint on purpose. Seven heavy lines read as a fitted
curve drawn badly, and the symbol has to be unmistakable next to greta.gam's
single fitted curve, which is the nearest thing to it in the family.

## The family

All four extension stickers share:

- the **greta wordmark at the original hex's size and height** — measured on the
rendered PNGs, the ink spans 0.362–0.622 of the image height against
`greta_hex.png`'s 0.362–0.623
- **Mulish** — Google renamed Muli to Mulish, so `font_add_google("Muli")` no
longer resolves, but it is the same face the original used
- the `light` theme — `#E0D2EE` fill, `#6B4A8B` type
- a hairline border, so the symbol is what distinguishes a sticker rather than
the frame
- a 2mm safe margin, verified against the rendered pixels rather than assumed

**greta's own hex is unchanged.** It stays the original at
[`logos/greta_hex.png`](https://github.com/greta-dev/greta/blob/main/logos/greta_hex.png);
these are designed to sit alongside it, not to replace it.

## To do

- [ ] copy the PNG into the package
- [ ] `usethis::use_logo("greta-gp.png")`
- [ ] check the logo renders in the README
- [ ] confirm pkgdown picks it up

Start by locating the package's existing logo assets and README rendering setup, then use the provided greta-gp.png from greta.hexes at png/light/stacked/greta-gp.png. Run usethis::use_logo("greta-gp.png"), check that the logo renders in the README, and confirm pkgdown picks it up.

Written by the indexing model from the issue text.