build: `clean-doc` target assumes Git

## Situation

The `clean-doc` target in the build assumes the system has access to Git:
https://github.com/canonical/sphinx-docs-starter-pack/blob/daed26d13711659d12cb9ba14f9e3923e3bd0b85/docs/Makefile#L100

While it's exceptionally rare that the host runtime won't have Git, we've experienced this in a project at Canonical. There are stable, POSIX-compliant ways to clean files.

## Request

Add a check like `which git`, and if not present, clean the doc files another way.

Inspect docs/Makefile at the linked clean-doc target, first checking how it currently relies on Git. Test the target in an environment without Git and compare its behavior with a POSIX-compliant cleanup approach. Done means documentation files are cleaned both when Git is available and when it is not.

Written by the indexing model from the issue text.