API: SQL class definitions renaming

WIP, related to #7960

- [x] renaming of 
  - PandasSQLAlchemy -> SQLDatabase
  - PandasSQLTable -> SQLTable
  - PandasSQLLegacy -> SQLiteDatabase
  - PandasSQLLegacyTable -> SQLiteTable
  - SQLDatabase.read_sql -> read_query
- [ ] adding docstrings to public functions of SQLDatabase
  - [x] read_query, read_table, to_sql
  - [ ] has_table, drop_table, ..
- [ ] add docs on use of SQLDatabase
- [ ] add api docs for SQLDatabase
- [x] rename some mentions of 'legacy' to 'fallback'
change here too