docs: add Keplars community email adapter

[debojyoti452]

Summary

Adds Keplars to the Community Email Adapters section in the email overview docs.

• Package: @keplars-hq/payload-email
• Keplars is a transactional email API with a priority-queue system (instant, high, async, bulk delivery tiers)
• Drop-in alternative to the Resend adapter

Usage

import { buildConfig } from 'payload'
import { keplarsAdapter } from '@keplars-hq/payload-email'

export default buildConfig({
  email: keplarsAdapter({
    apiKey: process.env.KEPLARS_API_KEY,
    defaultFromAddress: 'noreply@yourdomain.com',
    defaultFromName: 'Your App',
  }),
})

Full docs: https://docs.keplars.com/integrations/payload