Introduce a fast proxy mode to improve HTTP/1.1 performances with backends

Co-authored-by: Romain <rtribotte@users.noreply.github.com>
Co-authored-by: Julien Salleyron <julien.salleyron@gmail.com>

docs/content/user-guides/fastproxy.md

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+---
+title: "Traefik FastProxy Experimental Configuration"
+description: "This section of the Traefik Proxy documentation explains how to use the new FastProxy option."
+---
+
+# Traefik FastProxy Experimental Configuration
+
+## Overview
+
+This guide provides instructions on how to configure and use the new experimental `fastProxy` static configuration option in Traefik.
+The `fastProxy` option introduces a high-performance reverse proxy designed to enhance the performance of routing.
+
+!!! info "Limitations"
+
+    Please note that the new fast proxy implementation does not work with HTTP/2.
+    This means that when a H2C or HTTPS request with [HTTP2 enabled](../routing/services/index.md#disablehttp2) is sent to a backend, the fallback proxy is the regular one.
+
+    Additionnaly, observability features like tracing and OTEL semconv metrics are not supported for the moment.
+
+!!! warning "Experimental"
+    
+    The `fastProxy` option is currently experimental and subject to change in future releases. 
+    Use with caution in production environments.
+
+### Enabling FastProxy
+
+The fastProxy option is a static configuration parameter.
+To enable it, you need to configure it in your Traefik static configuration
+
+```yaml tab="File (YAML)"
+experimental:
+  fastProxy: {}
+```
+
+```toml tab="File (TOML)"
+[experimental.fastProxy]
+```
+
+```bash tab="CLI"
+--experimental.fastProxy
+```