Define the manifests path as a variable in `config.py`

### Why it needs to get done

to define the manifests path in one place and reuse it for all commands
it was first discussed in [this comment](https://github.com/canonical/data-science-stack/pull/43#discussion_r1517241851)

### What needs to get done

Move the definition of the manifests path to a variable in `config.py` and reuse this variable in all the commands `.py` files that interact with the manifests path.

in `config.py` do something like:
```
MANIFEST_TEMPLATES_LOCATION = Path("./manifest_templates")
ABSOLUTE_MANIFEST_TEMPLATES_LOCATION = Path(__file__).parent / MANIFEST_TEMPLATES_LOCATION
```

### When is the task considered done

manifests path definition is refactored as described and PR is merged