[improvement] provide a sample / walkthrough how to create a poor man's sources mirror

I have a computer that can't access internet from terminal, hence it can't download source package(s). I found a [page in docs](https://docs.conan.io/2/devops/backup_sources/sources_backup.html) describing how to use custom server to provide sources and wanted to try it with localhost.

From the docs it's unclear which file structure is expected, but fortunately web server logs gave insight on required file names'. But the last bit was missing: the contents of the JSON file. I managed to find it in [source code](https://github.com/conan-io/conan/blob/develop2/conan/internal/rest/download_cache.py): the smallest valid content is

```json
{"references": {}, "timestamp": }
```

I think it'd be very useful to provide a small description of the webserver file/directory structure as well as an example of JSON file.