Add programming example for micro-DMA function

Placeholder Mike Thompson
2023-02-10T03:05:24Z

### 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_