DOC: Improve documentation

[xmnlab]

Add more examples and explanations to the documentation.

References:

• https://sqlalchemy-ibmi.readthedocs.io/en/latest/
• https://github.com/druid-io/pydruid
• https://github.com/dropbox/PyHive#sqlalchemy
• https://github.com/aadel/sqlalchemy-solr
• https://github.com/blue-yonder/sqlalchemy_exasol
• https://github.com/preset-io/elasticsearch-dbapi/#using-sqlalchemy
• https://github.com/googleapis/python-bigquery-sqlalchemy
• https://pypi.org/project/ibm-db-sa/
• https://github.com/snowflakedb/snowflake-sqlalchemy
• https://pypi.org/project/teradatasqlalchemy/

References about integration with Apache Superset:

• https://thedataist.com/visualize-anything-with-superset-and-drill/

[tjbanks]

Great project overall! Thanks for your work!