Restore respect-gitignore to mado.toml once 0.4.0 is published

## What to do

Once 0.4.0 is published, restore `respect-gitignore` to `mado.toml`:

```toml
[lint]
respect-ignore = true
respect-gitignore = "repository-only"
output-format = "concise"
```

`"repository-only"` is the default, so this changes nothing about how the
repository is linted. `mado.toml` is the example configuration the README points
at, and it names every option; this is the one it is currently missing.

## Why it is out

#447 changed `respect-gitignore` from a boolean to a policy. The `Download` job
in `.github/workflows/ci-action.yml` runs the newest published release over this
checkout, so `mado.toml` has to be readable by 0.3.2 as well as by `main`, and
no spelling of the key satisfies both:

| `mado.toml` says | `main` | 0.3.2 |
|---|---|---|
| `respect-gitignore = true` | fails to load | loads |
| `respect-gitignore = "repository-only"` | loads | fails to load |

Once 0.4.0 is the newest release, the second row loads on both.

Nothing fails while the key is missing, which is why this is written down rather
than left to the next red build. #450 adds the procedure to `CONTRIBUTING.md`;
this issue is the instance of it that is currently outstanding.

## Related

- #447
- #448
- #450