doc: Make SD card peripheral docs more generic, fix one acronym

Fixes an issue in which "SDHC" had been incorrectly expanded to
"Secure Digital High Capacity" instead of "SD Host Controller".

Since this page is listed under "Peripherals" and since the
SD Host Controller API is not supposed to be used by applications,
the more generic and recognizable title "SD card interface" is used,
and users are pointed to pages relevant to them (disk access API
and SD card subsystem).

Signed-off-by: Egill Sigurdur <egill@egill.xyz>