Mention F10 in the menu to make this feature visible

Context:

I came across this issue on adding hotkey to open menu on headless machine that doesn't have a mouse.

https://github.com/microsoft/edit/issues/135

Key F10 was used to open menu, and issue was closed.

Problem:

F10 is great, but no where in the repo nor doc nor help in the edit itself mentions F10 can be used to open menu on machine that only have keyboard.

Suggestion:

There should be a text in the status bar saying something like "Press F10 to open menu".

Or at the center of page when edit command opened a brand new file, simialr to how VIM used to display a message when vim is started with just `vim`.

When user starts typing, the message should automatically disappear.

Illustration of the proposed help/tip message.

Search the repository for the existing F10 menu shortcut and for status-bar or empty-buffer message rendering. Decide where the keyboard hint belongs, make it disappear when typing begins, and verify the menu remains accessible through F10.

Written by the indexing model from the issue text.