doc: binman: Add guidance for determining ELF load addresses

doc: binman: Add guidance for determining ELF load addresses

Add documentation to help users understand how to determine where ELF
files will be loaded when using binman's 'fit,load' property. This
addresses the common confusion about how load addresses are determined
from ELF files.

The documentation explains three methods:
1. Using readelf to examine program headers
2. Checking the linker script (.lds file)
3. Using objdump to see section addresses

Also includes a specific example from binman tests showing how
elf_sections.lds sets ATF load address to 0x00000010 and
elf_sections_tee.lds sets TEE load address to 0x00100010 to avoid
memory overlap conflicts.

This helps users debug memory layout conflicts more efficiently when
working with FIT images containing multiple ELF components.

Signed-off-by: Aristo Chen <aristo.chen@canonical.com>