Document C code hardware drivers

[npetersen2]

Now that the C code drivers are maturing (i.e. ./drv/foo.c), we should make detailed documents about how to use them from a user perspective.

This should include:

1. Brief explanation of how the internals of the driver work. Should link to the IP core docs if appropriate.
2. Index of all available function calls and what they do.
3. Example code which exercises the driver.

[npetersen2]

Closing since this issue belongs in the new docs.amdc.dev.

I recreated the issue over there:

Severson-Group/docs.amdc.dev#13