Update description of github releases in docs. Resolves #3433. 

I also ended up rearranging docs/conf.py a bit.

# API Reference

```{eval-rst}
.. toctree::
   :maxdepth: 2

   api/js-api.md
   api/python-api.md
   Micropip API <https://micropip.pyodide.org/en/stable/project/api.html>
   api/pyodide-cli.md
   api/pyodide-build-cli.md
```

# API Reference

```{eval-rst}
.. toctree::
   :maxdepth: 2

   api/js-api.md
   api/python-api.md
   Micropip API <https://micropip.pyodide.org/en/stable/project/api.html>
   api/pyodide-cli.md
   api/pyodide-build-cli.md
```
