Reorganize docs (#8701)

Fixes #8671

Context
New layout as agreed offline

specs             (content remains same, we'll move here rar-core-scenarios from documentation/design folder)
 |-----proposed   (the secrets metadata and packages sourcing PRs would get routed here. In future - all in-progress work specs)
 |-----archive    (the rar-as-service.md from documentation/design will get moved here. In future - all not dismissed but not planned feature spec goes here)
Moving facilitated via git mv to preserve history and make diffs conscise

documentation/README.md

@@ -61,6 +61,7 @@ The folder contains collection of docs and references for MSBuild, detailed info
 * [Target maps](wiki/Target-Maps.md)
 * [Managing parallelism in MSBuild](specs/resource-management.md)
 * [SDK resolution](specs/sdk-resolvers-algorithm.md)
+* [RAR core scenarios](specs/rar-core-scenarios.md)
 
 ### Tasks
 
@@ -75,11 +76,14 @@ The folder contains collection of docs and references for MSBuild, detailed info
 * [Binary log](wiki/Binary-Log.md)
 * [Live logger: how to opt in](livelogger/Opt-In-Mechanism.md)
 
-## Designs
-
-* [Resolve Assembly Reference as a service](design/rar-as-service.md)
+## Archived Designs
+* [Resolve Assembly Reference as a service](specs/rar-as-service.md)
    * Prototype: https://github.com/dotnet/msbuild/issues/6193
 
+## Proposed Designs
+* [Packages Sourcing](specs/proposed/interactive-package-references.md)
+* [Secrets Metadata](specs/proposed/security-metadata.md)
+
 ## Community contributions
 
 * [MSBuild overview](Contributions/MSBuild-overview.md)