Add "warning box" to all sections detailing instructions with Polkadot-JS

[laboon]

As a first step, any section detailing how to do things with Polkadot-JS App should have a "warning box" above indicating that the Polkadot-JS App is focused on developers and advanced users. It should include a link the Wallets page and note that most functionality that a user would need is in one of them.

Ideally, these boxes would also indicating/linking specific alternative instructions, but this may take a while. I'd prefer to have the above as a stopgap measure, and then a second PR (or PRs) making these boxes more individualized per sections.

In the future, we should review all of our sections with PJS App and re-write if there are more user-friendly ways to do it. However, this is a much bigger process.

[laboon]

This could be similar to what the Support team is doing, i.e.:

Polkadot-JS UI is a web wallet meant for power users and developers. For everyday use, there are several user-friendly wallets funded by the Polkadot Treasury that support a plethora of features and platforms. Discover them on Polkadot's homepage.

[laboon]

Example