Complete the APB Advance Timer chapter of the User Manual

Placeholder Mike Thompson
2023-01-18T16:49:08Z

A (mostly empty) shell of the APB Advanced Timer peripheral exists at `docs/doc-src/ip-blocks/apb_adv_timer.rst`. This chapter needs to be updated such that the Reader understands _what_ the peripheral does, a high-level description of _how_ it works, and a description of how software should use the peripheral's CSRs to control its operation.

You are free to edit the chapter as you see fit, but a template has been provided with the following structure:

- **Features**: short description (can be a bullet list) of what the peripheral does.
- **Theory of Operation**: high-level description of the implementation (RTL).
- **Programming Model**: this section should answer the question: what does software need to do in order to configure/control/monitor the peripheral's function.
- **CSRs**: one table per CSR. An example is provided.

### Additional Information
A certain amount of this task will involve reverse engineering and hunting for information from disparate sources. A few suggestions:

- The [Pulp Platform](https://pulp-platform.org/) project and its associated GitHub repos.
- Device driver code in [core-v-mcu-cli-test](https://github.com/openhwgroup/core-v-mcu-cli-test).
- The RTL source code on GitHub ([link](https://github.com/openhwgroup/core-v-mcu/tree/master/rtl/apb_adv_timer)).