Move FAQ from Toolkit

[Yoshanuikabundi]

This PR re-organizes and replaces the Toolkit FAQ to be part of the top level docs.

Rendered: https://openff-docs--63.org.readthedocs.build/en/63/faq.html

Todo:

• [x] Add corrections from openforcefield/openff-toolkit#1848
• [x] Add section about warnings re: openforcefield/openff-toolkit#1932

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[Yoshanuikabundi]

@j-wags I've made some edits to what Matt brought up in openforcefield/openff-toolkit#1848, so I'm going to leave this unmerged for tonight in case he has corrections - I should be able to merge it in my AM before your talk.

@mattwthompson - sorry for dropping off the face of the planet and then asking for a quick response :/ I'm happy to merge this as is and you can get around to checking it at your leisure - I'm confident its correct, just not sure it addresses exactly the points you wanted to address.

[Yoshanuikabundi]

Do we have a way to remove parameters from a force field? We've got del in the FAQ:

del force_field['Constraints']["[#1:1]-[*:2]"]

But it doesn't work. Does anyone know if this is a change or did this just never work?

[mattwthompson]

I can get below to work with some effort, which I can't say would make me super happy as a user:

In [13]: sage = ForceField("openff-2.0.0.offxml")

In [14]: sage['Constraints']["[#1:1]-[*:2]"]
Out[14]: <ConstraintType with smirks: [#1:1]-[*:2]  id: c1  >

In [15]: sage["Constraints"].parameters.index(sage['Constraints']["[#1:1]-[*:2]"])
Out[15]: 0  # not relevant, just checking

In [16]: del sage['Constraints'].parameters["[#1:1]-[*:2]"]

In [17]: sage['Constraints']["[#1:1]-[*:2]"]
---------------------------------------------------------------------------
ParameterLookupError                      Traceback (most recent call last)
Cell In[17], line 1
----> 1 sage['Constraints']["[#1:1]-[*:2]"]

File ~/micromamba/envs/evaluator-test-env/lib/python3.11/site-packages/openff/toolkit/typing/engines/smirnoff/parameters.py:2439, in ParameterHandler.__getitem__(self, val)
   2434 def __getitem__(self, val):
   2435     """
   2436     Syntax sugar for looking up a ParameterType in a ParameterHandler
   2437     based on its SMIRKS.
   2438     """
-> 2439     return self.parameters[val]

File ~/micromamba/envs/evaluator-test-env/lib/python3.11/site-packages/openff/toolkit/typing/engines/smirnoff/parameters.py:1574, in ParameterList.__getitem__(self, item)
   1572     indexable_item: Union[int, slice] = item  # type: ignore[assignment]
   1573 elif isinstance(item, str):
-> 1574     indexable_item = self.index(item)
   1575 elif isinstance(item, ParameterType) or issubclass(type(item), ParameterType):
   1576     raise ParameterLookupError("Lookup by instance is not supported")

File ~/micromamba/envs/evaluator-test-env/lib/python3.11/site-packages/openff/toolkit/typing/engines/smirnoff/parameters.py:1529, in ParameterList.index(self, item, start, stop)
   1527     if parameter.smirks == item:
   1528         return self.index(parameter)
-> 1529 raise ParameterLookupError(f"SMIRKS {item} not found in ParameterList")

ParameterLookupError: SMIRKS [#1:1]-[*:2] not found in ParameterList

I think https://github.com/openforcefield/openff-toolkit/issues/1680 is related if we want to directly add this functionality to pass through ParameterHandlers - notably class ParameterList(list):, but other handy features allow users to bypass explicitly calling out .parameters