doc: add uplinks to Tezos docs & doc map

What

Add up links to the Tezos docs (at docs.tezos.com) and to the visual documentation map.

Why

Help users navigate in the documentation ecosystem, and clearly situate each piece. This should eliminate the sensation of being lost in a set of websites perceived as duplicates, when in fact they are complementary.

How

Add two icons in the left menu bar, just under the Octez logo.

Manually testing the MR

Check the doc artifact and see if it improves the navigation and helps understanding the current location.

Checklist

• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR