Add a documentaiton system

[LecrisUT]

For now the format would be plain markdown so that it is imported in packit.dev documentation. Template format is inspired from cmake's documentation:

# Title

<!-- SPHINX-START -->

Short description

## Synopsis
```
plans:
  import:
    url: https://github.com/packit/tmt-plans
    ref: main
    name: /plans/name-of-plan
```

## Options

`NAME_OF_ENV_INPUT` [Default: `some_value`]
  Details about option

## Examples

## See Also

<!-- SPHINX-END -->

• Synopsis is to make way when there is a clear way to interact with the imported plans
• Might also consider stories to be consistent with tmt's backend. Should be simple to move back-and-forth with a consistent documentation format. The only issue is that the documentation is not visible unless on the external site. Could make an action to generate and keep them in sync.