docs(urn): urn encoding

Checklist

[ ] The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
[ ] Links to related issues (if applicable)
[ ] Tests for the changes have been added/updated (if applicable)
[ ] Docs related to the changes have been added/updated (if applicable). If a new feature has been added a Usage Guide has been added for the same.
[ ] For any breaking change/potential downtime/deprecation/big changes an entry has been made in Updating DataHub