Updates references to keyboard keys in the help center docs to use
`<kbd>` HTML elements, which also means updating them to be as the
key would appear on a keyboard.
Previously, uppercase and lowercase letters were used to indicate
when/if the `Shift` key was being used, and even that was not
consistent throughout the documentation.
For CSS styling, adds a similar rule for `<kbd>` elements that is
used in `/static/styles/app_components.css`. And updates the CSS
class used in `/static/js/portico/help.js` for `adjust_mac_shortcuts`
accordingly.
Also, takes advantage of revising these pages for making small
updates for current help center documentation practices.
Updates the list of actions and buttons referenced in the help
center documentation for viewing images with lightbox.
Also, makes some minor corrections to the keyboard shortcut note.
Fixes#21527.