Docs: Document Inference Defaults in `quickstart.ipynb` and docstrings

[s3alfisc]

Context

Currently, the documentation is silent on standard error defaults - would be good to add notes to the feols() and fepois() docs.

In alignment with fixest, the behavior is the following:

• If no fixed effect is specified to be projected out, feols() and fepois() return "iid" standard errors.
• If one (or more) fixed effects are specified via formula syntax, inference is clustered by the first fixed effect (CRV1 inference).

feols("Y ~ X + C(f1)", data = data) # no fixed effect specified via two-part formula syntax -> iid inference
feols("Y ~ X | f1", data = data) # fixed effect specified via two-part formula syntax -> CRV1 inference clustered by f1
feols("Y ~ X | f1 + f2", data = data) # two fixed effects specified via two-part formula syntax -> CRV1 inference clustered by the first fixed effect f1

To do

• Add this information to the standard error section in the quickstart notebook.
• Add this information to the docstrings of the vcov of feols() and fepois().

[NKeleher]

@s3alfisc - I can pick this up as some initial docs work. I can likely get to it this week.