Beautify keyboard shortcuts modal

Description

We have a super helpful Keyboard shortcuts modal that displays all global and context-specific shortcuts within GitLab, accessible via a global Shift + ? shortcut.
Right now, this dialog lacks information structure and UI love.

Proposal

Let's improve the modal to reflect the new dialog design and provide better overview via improving more clear separation of categories and list items, following keyboard shortcut doc conventions and better descriptions.

There are four dialog variants depending on page context.

• Shortcuts modal from dashboard
• Shortcuts modal from issues
• Shortcuts modal from merge requests
• Shortcuts modal from wiki

Links / references

• Topic came up during last Product FGU when promoting the new Cmd + p shortcut in our Web IDE.