docs: document configurable SCIM webhook log retention

[pi1814]

Document the new DSYNC_WEBHOOK_LOGS_TTL environment variable for Ory Polis directory sync, which controls retention of SCIM webhook event logs. Adds a reference entry in the Directory Sync env vars section and a cross-reference from the Admin Portal webhook events logging page.

Related Issue or Design Document

Checklist

• 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.
• I have read the security policy.
• 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 security@ory.com) 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