[R2] Clarify Presigned URL docs

[harshil1712]

Existing documentation URL(s)

• https://developers.cloudflare.com/r2/api/s3/presigned-urls/

What changes are you suggesting?

Developers have reported issues understanding how to use Presigned URL for R2. One of the feedback we have got is the confusion on using Presigned URLs outside of the Workers environment. We need to audit the documentation and make any changes required.

From the developer:

Yeah, I did notice that after reading through the entire documentation and those linked pages. The confusing part is the *flow* of the documentation, not necessarily its contents. It went something like this:

1. Search for presigning of urls using R2, with the purpose being to let users upload from a browser, directly to R2
2. Find this page at the top of any kind of search result
3. Find a section dedicated to Workers, think "oh yeah, I'm using that - this is for me" (see image)
4. Become very confused and go down a rabbit hole of various issues, since this section, was, in fact, not for me

Another tidbit is that example here uses `aws4fetch` in the workers example, and none of the other examples use this client. The assumption then is that this is the only client that works with Workers, because:
- This is generally the case with fetch/web-compatible libraries compared to nodejs/other js ones
- The word "alternative" in the section title carries weight with it, signals that other approaches *do not work* when using Workers

I *think* thats the full extent of the confusion. Some of it is pedantic but may still be valuable to take a look over and see if you can clarify. I also think an example section dedicated to this use case (presign R2 url from worker, upload to it with cURL or something) could be really helpful, since it's not the first time someone has gone through the same process & issues as me here on Discord.

Additional information

One can find more information in this Discord thread: https://discord.com/channels/595317990191398933/1327764577630421002/1328414674642931786