Move the design proposals to a separate book

Fixes #3262

CONTRIBUTING.md

@@ -92,16 +92,13 @@ understood. Avoid introducing new 3-letter terms, which can be confused with 3-l
 Design Proposals
 ---
 
-Solana's architecture is described by a book generated from markdown files in
-the `book/src/` directory, maintained by an *editor* (currently @garious). To
-add a design proposal, you'll need to at least propose a change the content
-under the [Accepted Design
-Proposals](https://solana-labs.github.io/book-edge/proposals.html) chapter.
-Here's the full process:
+Solana's design proposals are part of a book generated from markdown files in
+the `proposals/src/` directory. To add a design proposal:
 
 1. Propose a design by creating a PR that adds a markdown document to the
-   directory `book/src/` and references it from the [table of
-   contents](book/src/SUMMARY.md). Add any relevant *maintainers* to the PR review.
+   directory `proposals/src/` and references it from the [table of
+   contents](proposals/src/SUMMARY.md). Add any relevant *maintainers* to the PR
+   review.
 2. The PR being merged indicates your proposed change was accepted and that the
    maintainers support your plan of attack.
 3. Submit PRs that implement the proposal. When the implementation reveals the