Add programming example for micro-DMA function

Task Description

Currently the micro-DMA chapter of the User Manual is a description of how the uDMA works. We need a programmer's view that explains how to write/read the uDMA and peripheral CSRs to transfer data to/from the L2 memories.

Let's start with a simple peripheral for this. I recommend either the uDMA UART or uDMA I2CM peripherals.

Description of Done

Merge pull-request attributed to this issue for docs/doc-src/udma_subsystem.rst plus the associated uDMA peripheral chapter (e.g. docs/doc-src/ip-blocks/udma_uart.rst).

Associated PRs

No response