[doc] Document retention RAM and boot services request

Nobody has claimed this yet.

### Description

At the moment the retention RAM is almost absent from yjr documentation: it is mentioned in the earlgrey datasheet and in the secure boot specification. It is in the memory map but that's easy to miss because it uses the same hardware IP as the RAM so unless one looks at the earlgrey memory map, it's impossible to find it just by looking at the hardware IP blocks for example.
Since the rentention RAM plays an important role for ROM/ROM_EXT requests (see [`BOOT_SERVICE_REQUEST`](https://opentitan.org/book/doc/security/specs/secure_boot/index.html?highlight=retention%20ram#boot-services-boot-services)), this should be documented somewhere.
Relevant code:
- [retention_ram.c](https://github.com/lowRISC/opentitan/blob/master//sw/device/silicon_creator/lib/drivers/retention_sram.c)
- [retention_ram.h](https://github.com/lowRISC/opentitan/blob/master/sw/device/silicon_creator/lib/drivers/retention_sram.h)

Relevant issues:
- #17340

Start with sw/device/silicon_creator/lib/drivers/retention_sram.h and retention_sram.c, then review the Earlgrey memory map and the secure boot specification's BOOT_SERVICE_REQUEST section. Document where retention RAM is located and how it relates to ROM/ROM_EXT boot service requests, with the result placed in an appropriate documentation location.

Written by the indexing model from the issue text.