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/content/docs/fa/config/panel.mdx

@@ -67,6 +67,7 @@ icon: SlidersHorizontal
 
 <Cards>
   <Card title="ربات Telegram" href="/docs/operations/telegram-bot" description="توکن، شناسه‌های چت، هشدارها و گزارش‌ها." />
+  <Card title="ربات Discord" href="/docs/operations/discord-bot" description="توکن، شناسه کانال و هشدارهای رویداد." />
   <Card title="اشتراک" href="/docs/config/subscription" description="سرور اشتراک، قالب‌ها و مسیرها." />
   <Card title="امنیت" href="/docs/operations/security" description="۲FA، محدودیت‌های IP و سخت‌سازی." />
 </Cards>