Direct users to add kv-namespaces to specific portion of toml

[EverlastingBugstopper]

cc @steveklabnik

If users add kv-namespaces snippet under a [site] config, the namespace is not parsed into a Wrangler Manifest so the namespace is still inaccessible to a worker.

account_id = "f5518bfbf88c230551a64797c221c7da"
name = "website"
type = "webpack"
route = "www.steveklabnik.com/*"
zone_id = "9359b254c46ff9f5c546203686794862"
workers_dev = false

[site]
bucket = "./public"
entry-point = "workers-site"

kv-namespaces = [
         { binding = "new_namespace", id = "5161cf32bf194293a8591c3c65a24c0a" }
]

We should do two things here:

1. If we detect kv-namespaces in the site struct, we should warn them to move it
2. When we ask people to add kv-namespaces to their toml, we should direct them to add it to the top level config, and not below site

Alternatively we could just rewrite their toml for them but that gets a little hairy, especially if they have things like comments

[gabbifish]

I think option 1 is by far the best option. Option 2 doesn't play well with folks who have different environments and don't necessarily want to put their kv-namespaces up in the top-level config.

[gabbifish]

Actually, could we even just make

[site]
bucket = ""
entry-point ""

into

site = {
    bucket = "", entry-point = ""
} 

instead? This way, there is no way kv-namespaces will be accidentally lumped under sites. We can also stop users from getting confused between sites and environments...