Add basthon launch button

[fortierq]

This PR adds a launch button for Basthon.

Basthon is a nice project to execute a notebook client-side (inside the browser), using Pyodide. Similar to Jupyterlite, but I found it more stable and usable with several languages (SQL, OCaml...).

To do

• [x] Get a square logo from @casatir
• [ ] Take another pass through the docs text
• [ ] Make sure it works as-expected

[fortierq]

Cleaned git history.

[choldgraf]

Found an image

Reviving this one from the dead :-D

I spoke with @casatir and he will create an SVG version of the b and the t from Basthon for use as a little square logo.

[casatir]

Hi there,

Thanks for using Basthon! Here it is.

[fortierq]

I tried to fix the tests, but after merging from the master branch, I get those errors :

E           sphinx.errors.ThemeError: An error happened in rendering the page index.
E           Reason: KeyError('https://google.com')

PS : @casatir , did you see my PR for Basthon (https://framagit.org/basthon/basthon-notebook/-/merge_requests/1) ?