Document the Backing Store Size experiment for heap snapshots in the Memory tool

[captainbrosset]

There's a new experiment in Edge 123 that makes the Shallow Size column in the Memory tool more useful. Many object types in JavaScript actually only store some metadata in the memory that they directly own. The contents of these object types is usually stored somewhere else. This makes the Shallow Size column a little less useful than it could be.

The new experiment makes it so that the Shallow Size counts everything for an object, both its metadata, and its contents, even if they're not stored at the same memory location.

This PR adds documentation for this.

Previews:

1. Memory terminology

   • Before
   • GitHub public preview
   • LMC preview

2. Record heap snapshots using the Memory tool

   • Before
   • GitHub public preview
   • LMC preview

AB#48902258

[captainbrosset]

Pinging @sethbrenith in case you want to review this.

[learn-build-service-prod[bot]]

Learn Build status updates of commit b4f56b9:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
microsoft-edge/devtools-guide-chromium/memory-problems/heap-snapshots.md	:white_check_mark:Succeeded	View
microsoft-edge/devtools-guide-chromium/memory-problems/heap-snapshots-images/customize-icon.png	:white_check_mark:Succeeded	View
microsoft-edge/devtools-guide-chromium/memory-problems/heap-snapshots-images/retainers-section.png	:white_check_mark:Succeeded	View
microsoft-edge/devtools-guide-chromium/memory-problems/heap-snapshots-images/settings-icon.png	:white_check_mark:Succeeded	View
microsoft-edge/devtools-guide-chromium/memory-problems/heap-snapshots-images/shallow-size-entire-object.png	:white_check_mark:Succeeded	View
microsoft-edge/devtools-guide-chromium/memory-problems/memory-101.md	:white_check_mark:Succeeded	View

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