Setting directories for static and docs

[psmulovics]

The directories in https://github.com/din0s/docusaurus-plugin-llms-txt/blob/master/src/index.js#L7-L8 should be configurable like they can be for other plugins, or being picked up from settings.

Eg my project has:

  "presets": [
    [
      "@docusaurus/preset-classic",
      {
        "docs": {
          "path": "../docs",
        },
...

This should be handled by this plugin. If helps, happy to submit PR for this.