docs: Add formulaic to ecosystem list (#2827)

* Add `formulaic` to ecosystem docs.

With Formulaic 1.2.0 released at long last, it's time to officially join the Narwhals ranks :).

* add to readme too

---------

Co-authored-by: Marco Gorelli <[email protected]>

Author: matthewwardrop

README.md

@@ -171,6 +171,7 @@ Join the party!
 - [altair](https://github.com/vega/altair/)
 - [bokeh](https://github.com/bokeh/bokeh)
 - [darts](https://github.com/unit8co/darts)
+- [formulaic](https://github.com/matthewwardrop/formulaic)
 - [hierarchicalforecast](https://github.com/Nixtla/hierarchicalforecast)
 - [marimo](https://github.com/marimo-team/marimo)
 - [metalearners](https://github.com/Quantco/metalearners)

docs/ecosystem.md

@@ -8,6 +8,7 @@ for their dataframe interoperability needs:
 * [altair](https://github.com/vega/altair/)
 * [bokeh](https://github.com/bokeh/bokeh)
 * [darts](https://github.com/unit8co/darts)
+* [formulaic](https://github.com/matthewwardrop/formulaic)
 * [hierarchicalforecast](https://github.com/Nixtla/hierarchicalforecast)
 * [marimo](https://github.com/marimo-team/marimo)
 * [metalearners](https://github.com/Quantco/metalearners)