chore: remove old documentation site (#7974)

This PR removes the old documentation overview site, as its content has
moved to the main Lean website infrastructure.

This should be merged when the new website section is deployed, after
installing appropriate redirects.

Developer documentation is remaining in Markdown form, but it will no
longer be part of the documentation hosted on the Lean website. Example
code stays here for CI, but it is now rendered via a Verso plugin.

Author: david-christiansen

.github/workflows/nix-ci.yml

@@ -2,25 +2,24 @@ This is the repository for **Lean 4**.
 
 # About
 
-- [Quickstart](https://lean-lang.org/lean4/doc/quickstart.html)
+- [Quickstart](https://lean-lang.org/documentation/setup/)
 - [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/)
-- [Documentation Overview](https://lean-lang.org/lean4/doc/)
+- [Documentation Overview](https://lean-lang.org/documentation/)
 - [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)
+- [Examples](https://lean-lang.org/documentation/examples/)
 - [External Contribution Guidelines](CONTRIBUTING.md)
-- [FAQ](https://lean-lang.org/lean4/doc/faq.html)
 
 # Installation
 
-See [Setting Up Lean](https://lean-lang.org/lean4/doc/setup.html).
+See [Setting Up Lean](https://lean-lang.org/documentation/setup/).
 
 # Contributing
 
 Please read our [Contribution Guidelines](CONTRIBUTING.md) first.
 
 # Building from Source
 
-See [Building Lean](https://lean-lang.org/lean4/doc/make/index.html) (documentation source: [doc/make/index.md](doc/make/index.md)).
+See [Building Lean](doc/make/index.md).

README.md

@@ -0,0 +1,10 @@
+# Developer Documentation and Examples
+
+This directory contains documentation that describes how to work on
+Lean itself, as well as examples that are included in documentation
+that's hosted on the Lean website.  The `make` directory contains
+information on building Lean, and the `dev` directory describes how to
+work on Lean.
+
+The [documentation section](https://lean-lang.org/documentation) has
+links to documentation that describes how to use Lean itself.