[SDK] Drop the landed plan documents and cut the migration document down

MediaBackendPlan.md and MemberMediaPlan.md described changes that have
landed. SdkMigration.md keeps only what moved from src/ into sdk/, with
the
paths as they are today. README.md gains the onboarding notes on the
three
API files, and its example uses createRTCSlot, which replaced
createMatrixRTCClient.

sdk/SdkArchitecture.md

@@ -10,8 +10,8 @@ interface.
 Element Call becomes one consumer of it: `CallViewModel` keeps what makes a session
 a _call_ (ringing, auto-leave, layout, sounds, reactions, settings, the host
 bridge) and takes everything else from the client. The dependency direction is
-therefore `Element Call → sdk`, never the reverse. The migration from today's
-`CallViewModel` to that shape is in [`SdkMigration.md`](./SdkMigration.md).
+therefore `Element Call → sdk`, never the reverse. What moved out of Element
+Call's `src/` to get there is listed in [`SdkMigration.md`](./SdkMigration.md).
 
 ## The architecture
 
@@ -570,9 +570,7 @@ and learns of remote members only through `mediaFor$`, registering each
 membership's transport for as long as the member's scope lives. Media keys reach
 it as a stream, `mediaKeys$`, replayed on subscribe; the js-sdk session never
 does. A host supplies its own backend through `MatrixRTCClientOptions.backend`;
-without one, LiveKit is built from the LiveKit-flavoured client options. The
-reasoning behind each of these choices is in
-[`MediaBackendPlan.md`](./MediaBackendPlan.md).
+without one, LiveKit is built from the LiveKit-flavoured client options.
 
 ### Authenticating with a LiveKit transport
 
@@ -769,7 +767,7 @@ sdk/
   src/utils/          mapScoped, network retry, display names, test helpers
   dev/                the harness (below)
   SdkArchitecture.md  this document
-  SdkMigration.md     how Element Call gets from CallViewModel to the client
+  SdkMigration.md     what moved from Element Call's src/ into the SDK
 ```
 
 The lint rule `element-call/sdk-import-boundary` keeps it that way: nothing under