[DOCS] Update Triangle Grain section

[genedan]

Description

The Triangle Grain section needs some updating. There is a missing grain level (Semiannual (S)), and some warnings appear after the two cells:

Is your feature request aligned with the scope of the package?

• Yes, absolutely!
• No, but it's still worth discussing.
• N/A (this request is not a codebase enhancement).

Describe the solution you'd like, or your current workaround.

• Update text to include semiannual grain (Semiannual (S)
• Fix the load_sample function for the quarterly data set so that it does not issue a warning.

Do you have any additional supporting notes?

The full warning text is:

/home/docs/checkouts/readthedocs.org/user_builds/chainladder-python/envs/stable/lib/python3.11/site-packages/chainladder/core/base.py:381: UserWarning: Could not infer format, so each element will be parsed individually, falling back to `dateutil`. To ensure parsing is consistent and as-expected, please specify a format.
  datetime_mapping = dict(zip(datetime_arg, pd.to_datetime(**date_inference)))