Refactor "Customise the setup"

## Background

The [Customise the setup](https://canonical-starter-pack.readthedocs-hosted.com/stable/how-to/customise/) how-to guide is currently a mix of mandatory steps and reference information.

## Request

Update this page to be a concise guide about what `config.py` parameters to change. Begin with "mandatory" parameters (e.g. name of the project), and continue with the minimal recommendations/best practices for a simple setup.

**It is not necessary to cover everything in the `conf.py`**. Detailed information about all the parameters will be part of a new reference page (https://github.com/canonical/sphinx-stack-docs/issues/6)

Consider renaming this page to "Configure the setup" or "Configure the documentation. I suggest using "configuration" to refer to the general act of tuning the `conf.py` file, and "customisation" in more specific and advanced contexts, like adding custom HTML templates or snippets of code.