docs(example.env): improve clarity of docker/example.env config for self-hosting (@nitin-is-me) (monkeytypegame#6777)

### Description

Improved the `docker/example.env` file to make it more beginner-friendly
and easier to configure for self-hosters.

Changes made:
- Added section headers for clarity (e.g. Required Config, Firebase
Config, etc.)
- Clarified which settings are optional (Firebase, email, ports)
- Added better inline comments explaining each env variable
- Improved formatting and spacing for better readability

---

### Checks

- [x] Make sure the PR title follows the Conventional Commits standard.
- [ ] Adding quotes? (N/A)
- [ ] Adding a language? (N/A)  
- [ ] Adding a theme? (N/A)  
- [ ] Adding a layout? (N/A)  
- [x] Check if any open issues are related – None that I found

---

### Closes

N/A

Author: nitin-is-me

docker/example.env

@@ -1,45 +1,44 @@
-#url of the frontend, this must be accessible by your clients browser
+# Copy this file to `.env` before starting the containers
+
+### === Required Config ===
+
+# URL of the frontend (accessible by browser)
 MONKEYTYPE_FRONTENDURL=http://myserver:8080
 
-#url of the backend server, this must be accessible by your clients browser
+# URL of the backend (accessible by browser)
 MONKEYTYPE_BACKENDURL=http://myserver:5005
 
-# firebase config
-# uncomment below config if you need user accounts
-#FIREBASE_APIKEY=
-#FIREBASE_AUTHDOMAIN=
-#FIREBASE_PROJECTID=
-#FIREBASE_STORAGEBUCKET=
-#FIREBASE_MESSAGINGSENDERID=
-#FIREBASE_APPID=
-
-
-# email server config
-# uncomment below if you want to send emails for e.g. password reset
-#EMAIL_HOST=mail.myserver
-#EMAIL_USER=mailuser
-#EMAIL_PASS=mailpass
-#EMAIL_PORT=465
-#EMAIL_FROM="Support <noreply@myserver>"
-
-# google recaptcha
-# uncomment below config if you need user accounts
-# you can use these defaults if you host this privately
-#RECAPTCHA_SITE_KEY=6LeIxAcTAAAAAJcZVRqyHh71UMIEGNQ_MXjiZKhI
-#RECAPTCHA_SECRET=6LeIxAcTAAAAAGG-vFI1TnRWxMZNFuojJ4WifJWe
+### === Optional: Google reCAPTCHA ===
+
+# Default keys below work for localhost/private instances
+# RECAPTCHA_SITE_KEY=6LeIxAcTAAAAAJcZVRqyHh71UMIEGNQ_MXjiZKhI
+# RECAPTCHA_SECRET=6LeIxAcTAAAAAGG-vFI1TnRWxMZNFuojJ4WifJWe
+
 RECAPTCHA_SITE_KEY=
 RECAPTCHA_SECRET=
 
-# use alternative ports
+### === Optional: Firebase ===
 
-# port of the frontend http server
-# HTTP_PORT=8080
+# Uncomment if using user accounts
+# FIREBASE_APIKEY=AIzaSy******** 
+# FIREBASE_AUTHDOMAIN=your-app.firebaseapp.com 
+# FIREBASE_PROJECTID=your-app 
+# FIREBASE_STORAGEBUCKET=your-app.appspot.com 
+# FIREBASE_MESSAGINGSENDERID=1234567890 
+# FIREBASE_APPID=1:1234567890:web:abcdef123456 
 
-# port of the backend api server
-# BACKEND_PORT=5005
+### === Optional: Email Server ===
+# Enables email (e.g. password reset)
 
-# port of the redis server, not exposed by default
-# REDIS_PORT=6379
+# EMAIL_HOST=smtp.mailserver.com 
+# [email protected] 
+# EMAIL_PASS=password
+# EMAIL_PORT=465
+# EMAIL_FROM="Support <[email protected]>"
+
+### === Optional: Custom Ports ===
 
-# port of the mongodb server, not exposed by default
+# HTTP_PORT=8080
+# BACKEND_PORT=5005
+# REDIS_PORT=6379
 # MONGO_PORT=27017