Installation Assistant: please merge /install and /install/os

Originally created by @Anonymous on #11493 (Redmine)

see https://labs.riseup.net/code/issues/11269#note-8

Feature Branch: doc/11493-merge-install-welcome

Related issues

• Related to #11269 (closed)
• Related to #11604 (closed)
• Related to #15597 (closed)
• Blocks #11494 (closed)
• Blocks #15392 (closed)