Update periodic execution docs

[katarinasupe]

The formatting and examples are not okay.
Make it more clear.
Link it properly from the DELETE and Import best practices pages.

https://memgraph.com/docs/querying/read-and-modify-data#periodic-execution

Other mistakes:
https://memgraph.com/docs/querying/clauses

Conclusion: full querying section needs to be reviewed and improved.