[Uid] Expand the explanation about the different UUID variants

[javiereguiluz]

I'm always in favor of making docs as concise as possible, but I think that in this case we should do the opposite and expand the existing docs a bit.

First, the explanation of UUIDs is a very big block of code: https://symfony.com/doc/current/components/uid.html#generating-uuids It looks intimidating and not that clear.

Second, we don't mention many details about some UUIDs, so I propose to add more details.

Lastly, the current examples don't work for UUIDv3 and v5. The given namespace argument must be a Uuid object, not a string:

// this does NOT work
$uuid = Uuid::v3(Uuid::NAMESPACE_DNS, $name);

// this WORKS:
$namespace = Uuid::fromString(Uuid::NAMESPACE_OID);
$uuid = Uuid::v3($namespace, 'Hello World');