Add new website to release notes.

Author: sam-mccall

clang-tools-extra/docs/ReleaseNotes.rst

@@ -32,6 +32,8 @@ infrastructure are described first, followed by tool-specific sections.
 Improvements to clangd
 ----------------------
 
+- clangd documentation is now found at https://clangd.llvm.org/
+
 - Go-to-definition, hover, find-references etc use a new mechanism to identify
   what is under the cursor, which is (hopefully) more consistent and accurate.