DOCS: Include `--lock-source-repo` option

Update the docs to include the new `--lock-source-repo` option introduced in this PR: https://github.com/github/gh-gei/pull/589

This option was added to both `migrate-repo` and `generate-script` commands.

This will make the source repo read-only as part of the migration. If the customer wishes to unlock a repo, they have to do it via the API (https://docs.github.com/en/rest/migrations/orgs#unlock-an-organization-repository). There is no way to unlock a repo via the GitHub UI or this CLI.

We have another issue in the backlog to add an `unlock-repo` command to this CLI at some point in the future. #593