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

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

[fortierq]

Cleaned git history.

[choldgraf]

I haven't used Basthon before but it seems reasonable to me. We might change up the way that launch buttons are added in the future but this seems fine to add in the meantime. One quick question about the logo image though

[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) ?