(#5849) Convert rst to markdown (#6040)

Converting some of the rst documentation to markdown.  Attempted to
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+# Overview
+Captcha can be enabled for this home server. This file explains how to do that.
+The captcha mechanism used is Google's ReCaptcha. This requires API keys from Google.
+
+## Getting keys
+
+Requires a public/private key pair from:
+
+<https://developers.google.com/recaptcha/>
+
+Must be a reCAPTCHA v2 key using the "I'm not a robot" Checkbox option
+
+## Setting ReCaptcha Keys
+
+The keys are a config option on the home server config. If they are not
+visible, you can generate them via `--generate-config`. Set the following value:
+
+    recaptcha_public_key: YOUR_PUBLIC_KEY
+    recaptcha_private_key: YOUR_PRIVATE_KEY
+
+In addition, you MUST enable captchas via:
+
+    enable_registration_captcha: true
+
+## Configuring IP used for auth
+
+The ReCaptcha API requires that the IP address of the user who solved the
+captcha is sent. If the client is connecting through a proxy or load balancer,
+it may be required to use the `X-Forwarded-For` (XFF) header instead of the origin
+IP address. This can be configured using the `x_forwarded` directive in the
+listeners section of the homeserver.yaml configuration file.