docs(capm3): add Metal3 custom resources docs

[pratik-mahalle]

Add Metal3 Custom Resources Documentation

Summary

This PR adds comprehensive documentation for Metal3 custom resources in the CAPM3 (Cluster API Provider Metal3) user guide. The documentation covers three key custom resources essential for managing bare metal infrastructure in Kubernetes clusters.

Changes Made

New Documentation Files Added

1. custom_resources.md - Overview and introduction to Metal3 custom resources
2. metal3data.md - Documentation for the Metal3Data resource
3. metal3datatemplate.md - Comprehensive guide for Metal3DataTemplate resource

Updated Files

• SUMMARY.md - Added navigation links for the new documentation sections

Documentation Coverage

Metal3DataTemplate

• Complete API reference with YAML examples
• Metadata specifications (strings, object names, indexes, IP pool references)
• Network data specifications following Nova network_data.json format
• Template reference management for versioning
• Usage guidelines and examples

Metal3Data

• Resource lifecycle and management
• Template rendering process
• Integration with Metal3DataTemplate

Custom Resources Overview

• Introduction to Metal3 custom resources
• Relationship between different resources
• Common use cases and patterns

Benefits

• Provides comprehensive reference for Metal3 users
• Includes practical examples for real-world scenarios
• Improves discoverability through proper navigation structure

---

Related Issue:
Fixes #398

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[metal3-io-bot]

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[pratik-mahalle]

Hey @lentzi90, have a look on this pr

[tuminoid]

Hi @pratik-mahalle !

Since your original PR, we've been aiming to CRD documentation per controller directly in the respective repo. I think currently we have such in IRSO and we have on-going PR in BMO. Instead of manually curating CRD documentation, we're using tooling to generate them.

I'd suggest actually doing the same for CAPM3 and IPAM, and then linking them here rather than manually recreating the same. It is not maintainable.

[pratik-mahalle]

Hey @tuminoid Thanks for the clarity! That makes sense, manually maintaining CRD docs can get messy pretty quickly.
but a quick question: how can I use the same tooling you mentioned to generate the CRD documentation for CAPM3 and IPAM? If you can point me to the scripts, configs, or an example setup (like the one in IRSO or the ongoing BMO PR), I can align with that workflow and update the links accordingly.

Happy to follow the standardized approach!

[tuminoid]

Just check the links I already provided. Also, I'm not @lentzi90 :)

[pratik-mahalle]

Sorry for the misunderstanding @tuminoid :)