feat: update .env.example with additional configuration options and documentation per env var

.env.example

@@ -1,31 +1,52 @@
+# Copy this file to .env and fill in the values before running the application.
+
+# The origin URL of the frontend application, used for CORS and constructing links
+# In production, set it to your frontend URL, e.g. https://esports.vash.software
+ORIGIN="http://localhost:5173"
+
+# The email of the root admin user, with global full permissions.
 ROOT_ADMIN_EMAIL=""
 
 # Docker Compose database password (used in docker-compose.yml)
 DB_PASSWORD=""
-
 DATABASE_URL="postgres://user:password@host:port/db-name"
 
-ORIGIN=""
+# Better Auth secret key, used for signing JWTs and other cryptographic operations.
-
-# Better Auth
 # For production use 32 characters and generated with high entropy
 # https://www.better-auth.com/docs/installation
 BETTER_AUTH_SECRET=""
 
-OSU_CLIENT_ID=""
-OSU_CLIENT_SECRET=""
-
-# Discord notifications
-DISCORD_BOT_TOKEN=""
-DISCORD_MATCH_CHANNEL_ID=""
-
 # Cloudflare R2 image storage
+# Create a new bucket and credentials at https://dash.cloudflare.com/r2
+# Make sure to set the bucket has public access
 R2_ACCOUNT_ID=""                                            # Your Cloudflare Account ID
 R2_ACCESS_KEY_ID=""                                         # R2 API token Access Key ID
 R2_SECRET_ACCESS_KEY=""                                     # R2 API token Secret Access Key
 R2_BUCKET="vash-esports-images"                             # Bucket name, e.g. vash-esports-images
 R2_PUBLIC_URL="https://cdn.esports.vash.software"           # Public URL for the bucket, e.g. https://cdn.esports.vash.software
 
+# osu!-related
+# Create an osu! API application here: https://osu.ppy.sh/home/account/edit#oauth
+OSU_CLIENT_ID=""
+OSU_CLIENT_SECRET=""
+
+# Discord-related
+# Create a Discord application here: https://discord.com/developers/applications
+DISCORD_CLIENT_ID=""
+DISCORD_CLIENT_SECRET=""
+DISCORD_BOT_TOKEN=""
+DISCORD_MATCH_CHANNEL_ID=""
+
+# GitHub-related
+# Create a GitHub OAuth app here: https://github.com/settings/developers
+GITHUB_CLIENT_ID=""
+GITHUB_CLIENT_SECRET=""
+
+# Google-related
+# Create a Google OAuth app here: https://console.cloud.google.com/apis/credentials
+GOOGLE_CLIENT_ID=""
+GOOGLE_CLIENT_SECRET=""
+
 # Sentry error tracking (https://sentry.io)
 PUBLIC_SENTRY_DSN=""                                        # DSN is write-only, safe to expose client-side
 SENTRY_AUTH_TOKEN=""                                        # Only needed in CI for source map uploads