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.

---------

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

README.md

@@ -37,7 +37,7 @@ Built as an enhanced fork of the original X-UI project, 3X-UI adds broader proto
 - **Multi-node support** — manage and scale across multiple servers from a single panel, including cloning inbounds onto other nodes.
 - **Outbound & routing** — WARP, NordVPN, PIA, custom routing rules, load balancers with balancer-to-balancer fallback, and outbound proxy chaining. Bundled geosite and geoip categories are browsable straight from the rule editor.
 - **Built-in subscription server** — raw, JSON, and Clash output, auto-selected from the client's User-Agent, plus [custom page templates](docs/custom-subscription-templates.md).
-- **Telegram bot** for remote monitoring and management.
+- **Telegram and Discord bots** for remote monitoring and management.
 - **RESTful API** with scoped, optionally expiring tokens and an in-panel API reference.
 - **Installable panel (PWA)** — pin 3X-UI to a desktop or phone home screen.
 - **Flexible storage** — SQLite (default) or PostgreSQL.