This help center article should include more features rather than just
focusing on the "go to conversation" button. We should broaden and
restructure this page to cover other advanced features.
Refactors the "Go to conversation" section as step-by-step instructions,
and adds a `keyboard_tip`.
Adds new section "Toggle between Ctrl+Enter and Enter".
Deletes the "Enable Enter to send" help center article, and adds its
content as a new subheading in this section.
Updates existing links accordingly and adds a URL redirect.
Documents "Enable Control + Enter to send".
Tweaks intro paragraph of "Mastering the compose box".
Fixes: #22817.
c6abb7bedf