update prax wallet guide

[redshiftzero]

The "Using Penumbra on the web" section is fairly out of date and could use an update. I propose taking a pass through and updating the copy/screenshots (or filling in the pages if they are stub pages):

• Installing (e.g. currently has some outdated info about selecting RPC endpoints and frontends)
• Viewing balances
• Shielding funds
• Swapping tokens
• Staking
• Governance
• Disconnect

I know there is WIP (e.g. #42) that may impact changes like #55 so wanted to open an issue first prior to making further PRs. Thoughts?