feat: add default values to attributes to documentation

## Current behavior

Given the schema entry:

```go
"rm": {
Type: schema.TypeBool,
Description: "If true, then the container will be automatically removed after his execution.",
Default: false,
Optional: true,
},
```

the rendered output is

```md
- **rm** (Boolean) If true, then the container will be automatically removed after his execution.
```

## Desired behavior

I'd like to have `Defaults to abc` in the generation as well as follows:

```md
- **rm** (Boolean) If true, then the container will be automatically removed after his execution. Defaults to `false`
```

## Versions
- terraform-plugin-docs: `v0.4.0`
- terraform-plugin-sdk/v2 `v2.6.1`