[docs] Document avoiding embedded structs in config structures

[ANAMASGARD]

Description

Add guidelines to coding-guidelines.md explaining why embedded structs should be avoided in configuration definitions. This helps prevent unmarshal issues, naming conflicts, and improves code clarity.

Key points added to the documentation:

• Unmarshal Compatibility: Embedded structs can break custom Unmarshal implementations
• Naming Conflicts: Embedded structs can cause field name collisions when multiple embedded types have fields with the same name
• Clarity: Named fields make the configuration structure more explicit and easier to understand
• Example code: Shows the difference between BAD (embedded) and GOOD (named fields) patterns

Link to tracking issue

Fixes #12719

Testing

Documentation only - no code changes. No testing required.

Documentation

Added new subsection "Avoid Embedded Structs" under the existing "Configuration structs" section in docs/coding-guidelines.md. The section includes:

• Clear explanation of what embedded structs are
• Three key reasons to avoid them in configurations
• Code example demonstrating the preferred approach using named fields with mapstructure tags

[dmathieu]

This kind of change doesn't need a changelog entry.

[dmathieu]

Is there a linter we could enable as well to enforce checking for this?

[evan-bradley]

There are cases where we want to achieve the same end-user functionality of embedding structs while still naming the field in the struct itself (example here). Can you document this as well?