Add Memory Inspection Information

Inspecting WebKit's memory usage is a complicated task, and we have numerous tools to support this process.

Started off with documenting the footprint command and Malloc Heap Breakdown build.

Author: stwrt

docs/Infrastructure/MemoryInspection.md

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+# Memory Inspection
+
+Tracking WebKit's memory usage is important to ensure we do not use excessive resources.
+The operating system (in combination with WebKit tools) provides numerous ways to inspect WebKit to discover where our memory is being allocated.
+
+## Build Settings
+
+### Malloc Heap Breakdown
+
+Malloc Heap Breakdown allows for fine-grained analysis of memory allocated per class. Classes marked with `WTF_MAKE_FAST_ALLOCATED_WITH_HEAP_IDENTIFIER(ClassName);` will be individually marked when using tools like `footprint`.
+
+To enable this build setting you need to flip two flags. One in `PlatformEnable.h` and the second in `BPlatform.h`.
+
+```cpp
+/* Source/WTF/wtf/PlatformEnable.h */
+
+/*
+ * Enable this to put each IsoHeap and other allocation categories into their own malloc heaps, so that tools like vmmap can show how big each heap is.
+ * Turn BENABLE_MALLOC_HEAP_BREAKDOWN on in bmalloc together when using this.
+ */
+#if !defined(ENABLE_MALLOC_HEAP_BREAKDOWN)
+#define ENABLE_MALLOC_HEAP_BREAKDOWN 0
+#endif
+```
+
+```cpp
+/* Source/bmalloc/bmalloc/BPlatform.h */
+
+/* Enable this to put each IsoHeap and other allocation categories into their own malloc heaps, so that tools like vmmap can show how big each heap is. */
+#define BENABLE_MALLOC_HEAP_BREAKDOWN 0
+```
+
+## Commands
+
+### Footprint
+
+Footprint is a macOS specific tool that allows the developer to check memory usage across regions.
+
+```shell
+> footprint WebKit
+Found process com.apple.WebKit.WebContent [27416] from partial name WebKit
+======================================================================
+com.apple.WebKit.WebContent [27416]: 64-bit    Footprint: 142 MB (16384 bytes per page)
+======================================================================
+
+  Dirty      Clean  Reclaimable    Regions    Category
+    ---        ---          ---        ---    ---
+ 108 MB        0 B        23 MB         11    WebKit malloc
+9664 KB        0 B          0 B         24    MALLOC_TINY
+6384 KB        0 B        16 KB          8    MALLOC_SMALL
+3904 KB        0 B          0 B        768    JS VM Gigacage
+...
+    ---        ---          ---        ---    ---
+ 142 MB      21 MB        23 MB       7001    TOTAL
+
+Auxiliary data:
+    dirty: N
+    phys_footprint_peak: 424 MB
+    phys_footprint: 142 MB
+```
+
+#### Results
+
+Refer to `man footprint` for a full guide on this tool.
+
+##### Dirty
+
+Memory that is written to by the process. Includes Swapped, non-volatile, and implicitly written memory.
+
+##### Clean
+
+Memory which is neither dirty nor reclaimable.
+
+##### Reclaimable
+
+Memory marked as available for reuse.
+
+##### Regions
+
+Number of VM Regions that contribute to this row.
+
+##### Category
+
+Descriptive name for this entry.