Assign myself for docs updates

This sets myself as the reviewer for docs submodule updates.
Now with rust-lang/triagebot#1673 automating
the process, this piece of the puzzle handles the assignment step.

Author: ehuss

triagebot.toml

@@ -568,6 +568,13 @@ fallback = [
 "/src/bootstrap" =                           ["bootstrap"]
 "/src/ci" =                                  ["infra-ci"]
 "/src/doc" =                                 ["docs"]
+"/src/doc/book" =                            ["@ehuss"]
+"/src/doc/edition-guide" =                   ["@ehuss"]
+"/src/doc/embedded-book" =                   ["@ehuss"]
+"/src/doc/nomicon" =                         ["@ehuss"]
+"/src/doc/reference" =                       ["@ehuss"]
+"/src/doc/rust-by-example" =                 ["@ehuss"]
+"/src/doc/rustc-dev-guide" =                 ["@ehuss"]
 "/src/doc/rustdoc" =                         ["rustdoc"]
 "/src/etc" =                                 ["@Mark-Simulacrum"]
 "/src/librustdoc" =                          ["rustdoc"]