Add roadmap 2021 to the docs site

This adds the roadmap entry in the site in the [1]:

• index menu on the left (prior to the components listing)
• main page under a Roadmap section just before the components section

Related to #3208 (closed)

• [1]

Test Plan

make -C docs happy.

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Migrated from D5422 (view on Phabricator)