Meshing doc updates

1. Create a legacy meshing node in the API reference.
a. Put tui + legacy mwf items there.
b. In the user guide, we should move the legacy workflows to the legacy user guide section. That's consistent with what we will do with the API reference. We won't even refer to it in meshing workflows.
c. Every node in the documentation should clearly explain the roles of all its direct children 
2. Make the two meshing_worklow pages adjacent under a single node. Clarify the relationships
between them in a parent page which is also the new landing page linked from the "meshing workflow" on the API reference landing page.
3. ~~Make the (Meshing)Workflow types directly constructible based on a meshing session context.
That will help clarify the meshing documentation and make it fully self-contained.~~
4. ~~Make the meshing contents hierarchy much more shallow. Remove datamodel altogether.~~
5. Remove all technical jargon from the docs where it cannot be justified.
```
Singleton -> group

Command -> command
```
6. Add API doc links from the user guide code.
7. ~~Consider a shared base class for the meshing sessions.~~
8. Rectify intellisense issues.

meshing
  meshing_worklow
    application
  general
  parts
  part_files
  task_object
  meshing_utilities
  preferences

legacy_meshing
  tui
  meshing
  workflow
  etc.

9. Re-word top-level user guide statement: "Workflows are divided into the new Meshing workflows and Classic meshing workflow, which align with the journal syntax."