Add documentation for the new `REVERSE_PROXY_LOGOUT_REDIRECT` setting in the `[security]` section, which allows redirecting users to an external URL or relative path after logout when using reverse proxy authentication.
For Gitea 1.26, ref https://github.com/go-gitea/gitea/pull/36085.
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This PR was written with the help of Claude Opus 4.6
Reviewed-on: https://gitea.com/gitea/docs/pulls/380
Reviewed-by: Lunny Xiao <[email protected]>
Co-authored-by: silverwind <[email protected]>
Co-committed-by: silverwind <[email protected]>