Rename `action` to `relation` in documentation

[sidwebworks]

Term action is misleading for new users - we should atleast rename it to relation instead in the docs

Related Issue or Design Document

https://www.ory.sh/docs/keto/guides/migrating-legacy-policies#rewriting-it-to-relationships

Checklist

• [x] I have read the contributing guidelines and signed the CLA.
• [ ] I have referenced an issue containing the design document if my change introduces a new feature.
• [x] I have read the security policy.
• [x] I confirm that this pull request does not address a security vulnerability. If this pull request addresses a security vulnerability, I confirm that I got approval (please contact [email protected]) from the maintainers to push the changes.
• [ ] I have added tests that prove my fix is effective or that my feature works.
• [ ] I have added the necessary documentation within the code base (if appropriate).

Further comments

[CLAassistant]

All committers have signed the CLA.

[aeneasr]

Thank you, this looks great! Could you please sign the CLA (just two clicks), then we can merge it right away!

[sidwebworks]

Hey @aeneasr I have signed it. Also can you please check your slack (https://ory-community.slack.com/archives/C010F7Z4XM1/p1719848117774519?thread_ts=1713859900.203939&cid=C010F7Z4XM1) there is another contribution I wanted to make though I am not sure how to continue with it.