Add docs for keep_user_credentials

[kuba2k2]

Description:

Pull request in esphome with YAML changes (if applicable): esphome/esphome#3813

Checklist:

• [x] Branch: next is for changes and new documentation that will go public with the next ESPHome release. Fixes, changes and adjustments for the current release should be created against current.
• [ ] Link added in /index.rst when creating new documents for new components or cookbook.

[netlify[bot]]

✅ Deploy Preview for esphome ready!

Name	Link
🔨 Latest commit	1a7484cbf8144dbb5327dc82eaf4fd64a6752f97
🔍 Latest deploy log	https://app.netlify.com/sites/esphome/deploys/63429aedd5ba54000883efd5
😎 Deploy Preview	https://deploy-preview-2301--esphome.netlify.app/components/captive_portal
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site settings.