Move docs from book/ to docs/ (#8469) (#8471)

automerge

docs/README.md

@@ -0,0 +1,26 @@
+Building the Solana book
+---
+
+Install dependencies, build, and test the docs:
+
+```bash
+$ ./build.sh
+```
+
+Run any Rust tests in the markdown:
+
+```bash
+$ make test
+```
+
+Render markdown as HTML:
+
+```bash
+$ make build
+```
+
+Render and view the docs:
+
+```bash
+$ make open
+```