[Docs]: Align documentation with the current package API

[rian-be]

Summary

Update the repository documentation so it matches the current codebase and namespace layout.

Goal

Remove drift between the docs and the actual public API.

Problem

The repository documentation still references older namespace names and older API shapes in some files, which makes the package harder to learn correctly.

Scope

• Update README.md to use current namespaces
• Update Docs/Getting-Started.md
• Update Docs/Basic-Usage.md
• Update Docs/Core-Concepts.md
• Update Docs/Architecture.md
• Rewrite Docs/API-Reference.md to match the current namespace layout
• Verify code snippets against the current public API

Design Expectations

• The docs should describe what the package actually exposes.
• Examples should use real namespaces and real entrypoints.
• The reference should be consistent with the current folder structure.

Acceptance Criteria

• Documentation no longer references removed namespaces such as Core.Features.*.
• The main getting started path uses ModularityKit.Context.Extensions.
• API reference pages map to the current code surface.

Non-Goals

• No marketing rewrite.
• No redesign of the docs site.
• No new runtime features.

Notes

This issue captures the remaining documentation work around the current code changes.