Add object lifetime and reference tracking documentation - #2460

Summary

Add a documentation guide describing how CsWinRT 3.0 manages the lifetime of projected Windows Runtime objects across the three systems involved: the .NET garbage collector, COM reference counting, and the XAML reference tracker.

Motivation

CsWinRT bridges three independent lifetime-management systems, and getting their interaction right is subtle (COM aggregation, tear-off interfaces, and reference-tracker cycles in particular). There was previously no conceptual reference for this in the 3.0 docs. This guide gives contributors an accurate mental model of how RCWs, CCWs, and reference tracking work, grounded in the current WinRT.Runtime2 implementation.

The content is adapted from the never-merged memory management doc (PR #899), which targeted CsWinRT 2.x and had accumulated unresolved review feedback. It has been rewritten to match the actual CsWinRT 3.0 runtime and to incorporate that feedback.

Changes

• docs/memory-management.md: new guide covering COM reference counting for RCWs (projected types deriving from WindowsRuntimeObject, with native objects owned by free-threaded and context-aware WindowsRuntimeObjectReference instances, plus GC memory pressure and agility), CCWs created through WindowsRuntimeComWrappers, COM aggregation for managed types extending unsealed Windows Runtime types, tear-off interface handling, and IReferenceTracker integration for XAML (including native/managed reference-cycle collection).
• README.md: link the new guide from the "Getting started" documentation list.

Notes

• All descriptions were verified against the WinRT.Runtime2 sources (the WindowsRuntimeObjectReference hierarchy, WindowsRuntimeObject, WindowsRuntimeComWrappers, and the object marshallers).
• The review comments left on PR Memory management doc #899 were revisited; the valid ones (API links, acronym introductions, sectioning, the tear-off recommendation, and grammar/style) are addressed in this rewrite.