docs: move devguide to sphinx for more powerful markup (#2898)

Having the devguide processed by Sphinx will let us use more powerful
markup, which
will help make it possible to create richer documentation.

docs/devguide.md

diff --git a/DEVELOPING.md b/docs/devguide.md
similarity index 98%
rename from DEVELOPING.md
rename to docs/devguide.md
index 83026c1..4d88b28 100644
--- a/DEVELOPING.md
+++ b/docs/devguide.md
@@ -1,4 +1,4 @@
-# For Developers
+# Dev Guide
 
 This document covers tips and guidance for working on the rules_python code
 base. A primary audience for it is first time contributors.

docs/index.md

diff --git a/docs/index.md b/docs/index.md
index 285b1cd..4983a6a 100644
--- a/docs/index.md
+++ b/docs/index.md
@@ -104,6 +104,7 @@
 REPL <repl>
 Extending <extending>
 Contributing <contributing>
+devguide
 support
 Changelog <changelog>
 api/index