Add docs for `download-logs` command

[synthead]

Introduced with https://github.com/github/gh-gei/pull/371.

Add documentation for the download-logs command.

From gei download-logs --help:

download-logs
  Downloads migration logs for migrations.

Usage:
  gei [options] download-logs

Options:
  --github-target-org <github-target-org> (REQUIRED)  Target GitHub organization to download logs from.
  --target-repo <target-repo> (REQUIRED)              Target repository to download latest log for.
  --target-api-url <target-api-url>                   Target GitHub API URL if not migrating to github.com 
                                                      (default: https://api.github.com).
  --github-target-pat <github-target-pat>             Personal access token of the GitHub target.  Overrides 
                                                      GH_PAT environment variable.
  --migration-log-file <migration-log-file>           Local file to write migration log to (default: 
                                                      migration-log-ORG-REPO.log).
  --overwrite                                         Overwrite migration log file if it exists.
  --verbose                                           Display more information to the console.
  -?, -h, --help                                      Show help and usage information