docs: add Discord bot to READMEs, architecture, operations guides, and locales (#6513)

* docs: add Discord bot to READMEs, architecture, operations guides, and locales

* docs: address review feedback on Discord bot formatting, backup commands, and architecture

* docs(discord): fix Persian typo and literal arrows on fa/zh bot pages

Senior review of #6513, two LOW findings in the two new pages:

- fa/operations/discord-bot.mdx:30 spelled "developers" with Cyrillic
  "де" in place of Persian "ده", rendering a mixed-script word.
- Both pages copied `$\rightarrow$` from the en page. The docs site has
  no math plugin (nothing in source.config.ts, no remark-math
  installed), so the built HTML shows the literal string
  "$\rightarrow$" in every menu path. Replaced with a Unicode arrow on
  fa and zh; en and ru have carried the same since #6486 and are left
  for a separate change.

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Co-authored-by: Sanaei <ho3ein.sanaei@gmail.com>

docs/README.md

@@ -43,7 +43,7 @@ The documentation walks you through 3x-ui from first install to day-to-day opera
 
 - **Getting Started** — installation, first login, and updating or uninstalling the panel.
 - **Configuration** — the panel, inbounds, REALITY, transports, clients, subscriptions, and share links.
-- **Operations** — reverse proxy, multi-node setups, outbounds & routing, backup/restore, the Telegram bot, and security.
+- **Operations** — reverse proxy, multi-node setups, outbounds & routing, backup/restore, Telegram and Discord bots, and security.
 - **Reference** — environment variables, the database, ports & firewall, and the HTTP API.
 - **Help** — troubleshooting, FAQ, migration, and how to contribute.