gitignore: add libuv book and GitHub template

PR-URL: https://github.com/nodejs/node/pull/13306
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

.gitignore

@@ -110,3 +110,8 @@ icu_config.gypi
 # Xcode workspaces and project folders
 *.xcodeproj
 *.xcworkspace
+
+# libuv book and GitHub template
+deps/uv/.github/
+deps/uv/docs/code/
+deps/uv/docs/src/guide/