Document Keil flash algo ELF section requirements

[flit]

The flash algorithm documentation does not explicitly document requirements for ELF sections; they are only documented implicitly through the flash algo template.

The assumption is that these ELF sections are required:

• PrgCode for all RO input sections (.text, .rodata)
• PrgData, zero or more for RW and ZI input sections (.data and .bss)
• DevDescr containing the FlashDevice symbol.

The requirements for the Keil flash algo format need to be documented thoroughly enough that someone can build an algo using any toolchain, starting from scratch.

These questions came up in relation to rp-rs/flash-algo#6, in the project for the Raspberry Pi Pico flash algo.