feat: add 'proof of React' challenge

Signed-off-by: Xe Iaso <me@xeiaso.net>

docs/docs/CHANGELOG.md

@@ -13,6 +13,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 <!-- This changes the project to: -->
 
+- Add a "proof of React" challenge to prove that the client is able to run a simple JSX app.
 - Downstream consumers can change the default [log/slog#Logger](https://pkg.go.dev/log/slog#Logger) instance that Anubis uses by setting `opts.Logger` to your slog instance of choice ([#864](https://github.com/TecharoHQ/anubis/issues/864)).
 - The [Thoth client](https://anubis.techaro.lol/docs/admin/thoth) is now public in the repo instead of being an internal package.
 - [Custom-AsyncHttpClient](https://github.com/AsyncHttpClient/async-http-client)'s default User-Agent has an increased weight by default ([#852](https://github.com/TecharoHQ/anubis/issues/852)).

docs/docs/admin/configuration/challenges/preact.mdx

@@ -0,0 +1,19 @@
+# Preact
+
+The `preact` challenge sends the browser a simple challenge that makes it run very lightweight JavaScript that proves the client is able to execute client-side Javascript. It uses [Preact](https://www.npmjs.com/package/preact) (a lightweight client side web framework in the vein of React) to do this.
+
+To use it in your Anubis configuration:
+
+```yaml
+# Generic catchall rule
+- name: generic-browser
+  user_agent_regex: >-
+    Mozilla|Opera
+  action: CHALLENGE
+  challenge:
+    difficulty: 1 # Number of seconds to wait before refreshing the page
+    report_as: 4 # Unused by this challenge method
+    algorithm: preact
+```
+
+This is the default challenge method for most clients.