docs: add sdcard docs

docs/badge/sdcard.rst

+SD Card
+-------
+
+There's a microSD card slot on the inner side of the badge.
+
+There's no first-class firmware support for it yet, but you can mount it in the
+micropython VFS with:
+
+.. code-block:: python
+
+   import machine, os
+   sdcard = machine.SDCard(clk=47, cmd=48, d0=21)
+   os.mount(sdcard, "/sd")
+
+Filesystem support is provided by `FatFs
+<http://elm-chan.org/fsw/ff/00index_e.html>` which claims to support
+FAT16/32/exFAT. TODO: test
+

docs/index.rst

@@ -12,6 +12,7 @@ Welcome to flow3r's documentation!
 
    badge/hardware_specs.rst
    badge/badge_link.rst
+   badge/sdcard.rst
 
 .. toctree::
    :maxdepth: 1