Use Sphinx `:doc:` roles for links to docs.aiven.io documents

[TibsAtWork]

We used to have to be more cautious about using :doc: because Vale would spell check the document names.

Now that's not an issue, we could look for all the https://docs.aiven.io links and consider converting them to :doc: links.

Why is this worth doing? Mainly because it makes it clearer that the links are into our own documentation. It also reduces the length of the link, making it easier to read the source text.