Browser client cert documentation

[tashian]

• Need new instructions for setting up the Browser resource
• Key protection for Linux browsers needs to be set to None for the Browser account.
• Is the agent required? (I think so?)
• Should we add step-agent-plugin doctor instructions for troubleshooting?
• Which root CA should clients trust? The Accounts CA? Why not the workloads CA?

for macOS

• Review FIrefox & Chrome instructions
• Clarify what the agent does with regard to Keychain identity preferences

for Linux

• Yee's doc instructs the user to check the agent log to find the path to the root CA certificate, then to add the root to the trust store using step certificate install. What's a simpler and more reliable way to find the path to the CA certificate?
• Add Firefox

for Windows

• add Chrome & Edge