docs: add Hosting your own Chat Relay guide (#7272)

* docs: add Hosting your own Chat Relay guide

* cli: run headless relay without a terminal

--headless (no -e) now runs via simplexChatCore instead of the terminal
UI, so the relay runs as a systemd service without a TTY (withTerminal
requires one). Drains the event queue logging only errors, and sets
stdout to line-buffering so logs reach the journal.

* docs/chat-relay: debounce service

* Apply suggestions from code review

Co-authored-by: Evgeny <evgeny@poberezkin.com>

---------

Co-authored-by: Evgeny <evgeny@poberezkin.com>

website/src/_data/docs_sidebar.json

@@ -28,6 +28,7 @@
                 "TRANSLATIONS.md",
                 "WEBRTC.md",
                 "XFTP-SERVER.md",
+                "CHAT-RELAY.md",
                 "DOWNLOADS.md",
                 "REPRODUCE.md",
                 "TRANSPARENCY.md",