docs(logs): explain log drain flow

[vaguul]

Changes

• Explain the two required parts of log drains: server-level Fluent Bit container and resource-level logging driver changes.
• Document that drained resources send logs to tcp://127.0.0.1:24224 through Docker's fluentd logging driver after restart.
• Add a minimal forward input example for custom Fluent Bit configurations.

Issue

• Closes Improve: explain what how log drain works #288

Verification

• Checked Coolify's StartLogDrain action that starts coolify-log-drain and binds 127.0.0.1:24224:24224.
• Checked generate_fluentd_configuration() and application deployment code that injects the fluentd logging driver when both server and resource log drains are enabled.

Testing

• git diff --check
• Not run: local docs build/typecheck, because this checkout has no bun command or node_modules installed.