wireguard-client-mullvad: clarifications, wording, and formatting

[sburris0]

Hi! I ended up needing to run through this guide, so I thought I'd do an editing pass while following it to clarify parts I found confusing.

Notable content changes:

• Guide steps
  • Split WireGuard peer setup into its own step
  • Assignments and Routing -> Configure NAT (more descriptive/specific)
  • Add conclusion step
• Step 1
  • curl strings -> curl commands
• Step 2
  • Clarify where 51820 port number comes from (WireGuard default)
  • Clarify that "Domain name" from Mullvad's server list is used for Endpoint address
  • Remove going back to instance tab to map instance <-> peer, as already-open peer dialog has a dropdown for this
  • "Now we can Enable the VPN" -> "Now we Enable the VPN" (make it clear the user is supposed to do it here)
• Step 3
  • Remove duplicate "add a rule", mention rule generation must be manual/hybrid BEFORE adding a rule

Notable formatting changes:

• Make capitalization of buttons match web gui
• Consistent WireGuard capitalization