chore: Simple app data config

[fedgiac]

Background

App data has a lot of logic built on top and new users can find the whole thing inscrutable at first.
Nothing in the app data is too important unless the user has specific needs.
It would be good if they could just ignore the whole thing, at least at first when playing around with CoW Protocol.

Details

I'd like the main reference page for app data (which I believe is this) to include a section that states something like "if you don't care, just use this app data hash: 0x1337...".
The specific hash could be many things:

• 0x00..00, cheaper and afaik supported by the backend.
• hash({}), explicitly saying that we don't care about any field.
• hash({"appCode":"CoW Docs"}), to let us know that this was taken from the docs.

It would also be very good to link to PUT /api/v1/app_data rather than asking users to upload to IPFS themselves.

Acceptance criteria

• Add instructions on the app data page that allows a user to completely ignore what the app data is.