Adds Certbot document to docs site

docs/src/certbot/index.md

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+---
+outline: deep
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+
+## Certbot DNS plugins in Nginx Proxy Manager
+
+Nginx Proxy Manager uses Certbot to issue and renew Let’s Encrypt certificates.
+
+When you request a certificate using a DNS challenge, Nginx Proxy Manager installs
+the corresponding Certbot DNS plugin for the provider you selected. The available
+providers and package versions are defined in
+⁠[certbot-dns-plugins.json](https://github.com/NginxProxyManager/nginx-proxy-manager/blob/develop/backend/certbot/dns-plugins.json).
+
+## Important limitations
+
+DNS plugins are maintained independently from Certbot and from Nginx Proxy Manager. As a result:
+- Some plugins may lag behind current Certbot or dependency versions.
+- A plugin may install package versions that conflict with other Certbot components.
+- Support quality varies between providers, and not every plugin is regularly tested in Nginx Proxy Manager.
+
+::: warning
+Using more than one DNS provider in the same Nginx Proxy Manager instance may introduce Python
+dependency conflicts between Certbot plugins.
+:::
+
+## If a DNS plugin does not work
+
+1. Check the Nginx Proxy Manager container logs for Certbot or Python package installation errors.
+2. Identify the plugin package and version defined for your provider in
+⁠[certbot-dns-plugins.json](https://github.com/NginxProxyManager/nginx-proxy-manager/blob/develop/backend/certbot/dns-plugins.json).
+3. Check [PyPI](https://pypi.org/) or the plugin project for a newer compatible release.
+4. If needed, update the plugin definition, including any required dependency pins.
+5. Submitting a pull request and CI will build a testable docker image for you.
+
+## File reference
+
+Use this file when reviewing or updating provider definitions:
+- ⁠[certbot-dns-plugins.json](https://github.com/NginxProxyManager/nginx-proxy-manager/blob/develop/backend/certbot/dns-plugins.json)