chore: update documentation title and link README to reference (#6409)

This PR makes the title of the mdbook documentation page reflect the
updated link on lean-lang.org and adds the reference manual to the
README.

README.md

@@ -6,7 +6,8 @@ This is the repository for **Lean 4**.
 - [Homepage](https://lean-lang.org)
 - [Theorem Proving Tutorial](https://lean-lang.org/theorem_proving_in_lean4/)
 - [Functional Programming in Lean](https://lean-lang.org/functional_programming_in_lean/)
-- [Manual](https://lean-lang.org/lean4/doc/)
+- [Documentation Overview](https://lean-lang.org/lean4/doc/)
+- [Language Reference](https://lean-lang.org/doc/reference/latest/)
 - [Release notes](RELEASES.md) starting at v4.0.0-m3
 - [Examples](https://lean-lang.org/lean4/doc/examples.html)
 - [External Contribution Guidelines](CONTRIBUTING.md)

doc/book.toml

@@ -3,7 +3,7 @@ authors = ["Leonardo de Moura", "Sebastian Ullrich"]
 language = "en"
 multilingual = false
 src = "."
-title = "Lean Manual"
+title = "Lean Documentation Overview"
 
 [build]
 build-dir = "out"