docs: add guide for rewrites

[hperl]

This PR adds documentation for the userset rewrites.

It is blocked by ory/keto#877, but the content can already be reviewed.

[hperl]

/cc @zepatrik @tomekpapiernik

[tomekpapiernik]

These are some nicely written documents :) Please have a look at the specific comments I left and address them.

One general remark is that it would be awesome if you could get rid of the "we" form when describing subsequent steps of some process. You're creating instructions that the user has to follow to get certain results, so it's totally okay to use imperative forms, for example:

We use the following relation tuples to showcase the namespace configuration.
turn it into
These relation tuples showcase namespace configuration.

There's no need for this undefined "we" in the docs ;) I hope you know what I mean. Let me know if you need any help! I'll be happy to take another look, please let me know when you're ready.

[hperl]

Hi @tomekpapiernik, I addressed all comments :).

[tomekpapiernik]

Looking better now, still a few more things left to fix though ;)

[aeneasr]

The feature has long been shipped and the PR is open for a month, I will get this over the finish line. Please make the minor language changes which are not blocking in a follow up PR.