feat: Add Nagios monitoring provider

[akachi10]

What this PR does

Adds a new Nagios provider for Keep, allowing users to receive and manage Nagios alerts.

Features

• Support for both Nagios Core (via JSON CGI) and Nagios XI (via REST API)
• Webhook integration for real-time alert forwarding
• Maps Nagios host/service states to Keep alert model
• Handles notification types (PROBLEM, RECOVERY, ACKNOWLEDGEMENT, DOWNTIME)

Files

• keep/providers/nagios_provider/nagios_provider.py - Provider implementation
• docs/providers/documentation/nagios-provider.mdx - Documentation

Testing

• Tested webhook payload formatting with sample Nagios notification data
• Verified API integration patterns match existing providers (Icinga2, Zabbix)

Closes #3960

[vercel]

@akachi10 is attempting to deploy a commit to the KeepHQ Team on Vercel.

A member of the Team first needs to authorize it.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[greptile-apps]

Target branch is not in the allowed branches list.