doc: move style guide to findable location

Move the style guide to doc/README.md so people might find it. The
current location is easily overlooked, as is evidenced by
#41025

Author: Trott

doc/guides/doc-style-guide.md renamed to doc/guides/README.md

@@ -121,7 +121,7 @@ If you are modifying code, please be sure to run `make lint` (or
 code style guide.
 
 Any documentation you write (including code comments and API documentation)
-should follow the [Style Guide](../doc-style-guide.md). Code samples
+should follow the [Style Guide](../../README.md). Code samples
 included in the API docs will also be checked when running `make lint` (or
 `vcbuild.bat lint` on Windows). If you are adding to or deprecating an API,
 add or change the appropriate YAML documentation. Use `REPLACEME` for the