docs(cloudflare): minor updates to the caching doc

[vicb]

• A queue is only needed for time base revalidation
• Add docs about CACHE_PURGE_API_TOKEN and CACHE_PURGE_ZONE_ID

[conico974]

I think i know why there was a mention of On-demand, res.revalidate use the same binding that the one used by the queue. We only talk about it in the queue part of the docs

[vicb]

Do you suggest to update the the doc as:

-A queue must be setup for projects using Time-Based revalidation.
+A queue must be setup for projects using Time-Based revalidation or `res.revalidate` (Page Router)
It is not needed when revalidation is not used nor only On-Demand revalidation is used.

[conico974]

I guess we could put it here, but it's not actually the queue that is needed it's only the binding itself.
Maybe we don't need the fix anymore actually, since now fetch are public if i'm not mistaken (We had to use the binding, but now it can just be a fetch, which i think is the default)

[vicb]

We had to use the binding, but now it can just be a fetch, which i think is the default

The binding will always be faster than a public fetch as it will skip part of the Cloudflare infra.
So we decided to keep using the binding for internal code to benefit for the improved perf.

I'll double check what we do currently.