support theme URLs that end in .release

[parmentelat]

trafficstars

implement an idea described in https://github.com/jupyter-book/mystmd/issues/2317

specifically, this allows a theme endpoint to define a 'download' key as being e.g.

"download": "https://github.com/jupyter-book/myst-theme/book.release"

and then rewrite it into

https://github.com/jupyter-book/myst-theme/releases/download/main/book.zip