LibreTranslate options are not clearly documented

In the docs it says > LibreTranslate needs `LIBRE_TRANSLATE_API_KEY` and `LIBRE_TRANSLATE_ENDPOINT` [Source](https://github.com/mastodon/documentation/blob/5c8597039a7ce1acb0941a5331d34d412e2613fd/content/en/admin/config.md?plain=1#L784) This should be improved in several ways: 1. `LIBRE_TRANSLATE_API_KEY` is in fact not required. LibreTranslate can be run without API keys. In fact, the devcontainer setup of mastodon/mastodon gives an example for this. 2. There should be an example for `LIBRE_TRANSLATE_ENDPOINT` to illustrate whether the /translate path should be included or not (it shouldn't).