Fix broken links, style consistency

 - Fixed link to firmware repo
 - Link to `/otp/main.c` now links to file instead of parent directory
 - `builtin` not `built-in`
 - I took the word "function" out of the link to `l_hw_digital_write` because the link to `hw_digital_write` below had the word "function" outside the link. Pedantic but whatever. :-)