Exposure & User guide of `using` context manager

- Must be usable through `ansys.fluent.core` but is currently esposed through `ansys.fluent.core.solver` (implemented in utils)
- Needs to be documented in the user guide for general purpose since it is usable for meshing and solver as shown by the tests
  - Place it anywhere after Meshing mode sessions in https://fluent.docs.pyansys.com/version/stable/user_guide/session/session.html