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>

docs/content/docs/en/guide/index.mdx

@@ -40,7 +40,7 @@ flowchart LR
 - **Per-client** traffic quotas, expiry dates, IP limits, online status, and
   one-click share links / QR codes.
 - **Subscriptions** in VLESS, Clash/Mihomo, and JSON formats.
-- Operational tooling: **multi-node** management, a **Telegram bot**, backups,
+- Operational tooling: **multi-node** management, **Telegram and Discord bots**, backups,
   Fail2ban-based IP limiting, and a documented REST API.
 
 ## Under the hood