doc: design: Add updated information on data sharing

this, the target must provide a definition for the `boot_save_shared_data()`

this, the target must provide a definition for the `boot_save_shared_data()`

function which is declared in `boot/bootutil/include/bootutil/boot_record.h`.

function which is declared in `boot/bootutil/include/bootutil/boot_record.h`.

The `boot_add_data_to_shared_area()` function can be used for adding new TLV

The `boot_add_data_to_shared_area()` function can be used for adding new TLV

entries to the shared data area.

entries to the shared data area. Alternatively, setting the

`MCUBOOT_DATA_SHARING_BOOTINFO` option will provide a default function for

this which saves information such as the maximum application size, bootloader

version (if available), running slot number, if recovery is part of MCUboot

and the signature type. Details of the TLVs for this information can be found

in `boot/bootutil/include/bootutil/boot_status.h` with `BLINFO_` prefixes.

## [Testing in CI](#testing-in-ci)

## [Testing in CI](#testing-in-ci)