Create CODEOWNERS (#21045)

Added CODEOWNERS file, made myself the owner of the docs directory.

.github/CODEOWNERS

+# Lines starting with '#' are comments.
+# Each line is a file pattern followed by one or more owners.
+
+# More details are here: https://help.github.com/articles/about-codeowners/
+
+# The '*' pattern is global owners.
+
+# Order is important. The last matching pattern has the most precedence.
+# The folders are ordered as follows:
+
+# In each subsection folders are ordered first by depth, then alphabetically.
+# This should make it easy to add new rules without breaking existing ones.
+
+# Documentation owner: Diana Payton
+/docs/ @oddlittlebird
\ No newline at end of file