Add examples for AQL string functions (#801)
Add examples for AQL string functions (#801)

* Add examples for SUBSTRING, UPPER and LOWER

* Add examples for TO_BASE64, TO_HEX and UUID too

Co-authored-by: Jana Spiller <080e99b5d907dd9812b23330d98aa09a66be725d@users.noreply.github.com>
arangodb/docs,arangodb/docs,arangodb/docs,arangodb/docs