DOC: Include missing holiday observance rules

[Jeitan]

Location of the documentation

Timeseries Holiday section in doc/source/user_guide/timeseries.rst.

Documentation problem

The list of available holiday observance rules is incomplete - several available observances are missing.

Suggested fix for documentation

The following observances should be added to the documentation because they are already available:

• next_workday
• previous_workday
• before_nearest_workday
• after_nearest_workday

Before I examined holiday via TAB-completion I had written my own version of weekend_to_monday; others may likewise re-invent these wheels if they're not documented.

(Updated to remove "weekend_to_monday" because it does the same thing as "next_monday")

[Jeitan]

I was poking at this, and discovered that weekend_to_monday actually does the same thing that next_monday does. I think I had taken "next Monday" too literally and thought that that would move even other weekdays to Monday. However, the code is actually the same (except for the order in which the weekday is checked). So one of them should probably go, and whichever one survives live in the documentation (personally, I think "weekend_to_monday" is clearer, but I could see wanting to complement previous_friday).
(The remaining 4 should still be documented)

[leonarduschen]

pandas/pandas/tseries/holiday.py

Line 15 in da5f914

def next_monday(dt: datetime) -> datetime:

We should probably remove next_monday since it's kinda misleading as it doesn't do anything if the current day is not Saturday or Sunday.

[leonarduschen]

There are room for improvement for the docstring as well. For example:

Docstring of before_nearest_workday:

pandas/pandas/tseries/holiday.py

Line 111 in 65319af

returns previous workday after nearest workday

Docstring of after_nearest_workday:

pandas/pandas/tseries/holiday.py

Line 118 in 65319af

returns next workday after nearest workday

[dontgoto]

take