Add way to link to complementary docs

[djwfyi]

We need to add a way to link to complementary products from the MinIO docs.

We are not planning to add links to the top nav across min.io sites.

• Modify the Hugo theme to add a "Go To" dropdown as on slides 4 and 5 of the mockup
• Have an easy way to modify the list of doc sites (should just be MinIO and KES to start)