Initial command palette docs

What does this MR do and why?

It adds basic documentation for the command palette. It might not be in the right location but it should give the TW the initial direction and description of the current capabilities if the command palette feature.

Note, that command palette functionality is available in the Global Search modal and is not a separate thing. To enable the command palette mode, start the search by one of the special chars. Instructions below.

How to set up and validate locally

👣 Enabled the feature flag

To test the command palette functionality

rails c 
Feature.enable(:command_palette)

👣 Click on the Global Search icon in the new navigation (or / shortcut)

👣 Start @user, ^project or use > to execute some default commands or jump to one of the menu items. In project scope you can also jump to project file when input starts with /.