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.
Lauryn Menard
Alya Abbott