docs: Specify the semantics of empty Series aggregations

[coastalwhite]

@rodrigogiraoserrao I cannot really build the docs on my laptop. Could you do a grammar and spellcheck and you could verify that this renders correctly?

[rodrigogiraoserrao]

Preferred:

Suggested change

	## Behavior with empty `Series`
	## Aggregations on empty series

But this might do as well:

Suggested change

	## Behavior with empty `Series`
	## Behavior with empty series

[rodrigogiraoserrao]

(A sentence per line is good because it makes diffs cleaner and makes it easier to review the docs.)

“Consequently, .group_by().agg() on columns with null values might result in different results than would be given by an SQL ”

The table shows results for aggregations computed on empty series.
What do empty series have to do with series that contain null values?

[rodrigogiraoserrao]

You used the word “semantics” 4 times in the first 3 sentences and that's quite a heavy word for a user-friendly user guide.
Here's a possible rewrite in simpler English:

When computing aggregations on empty series, Polars tries to follow set theory and Python's behaviour.
This differs from SQL for some operations: for example, pl.Series([], pl.Int32).sum() is equal to 0 in Polars but it is NULL in SQL.
Below we provide an overview of all aggregations and the return value when performed on an empty series.

[rodrigogiraoserrao]

I can't find the method get on a series:

>>> import polars as pl
>>> pl.Series([]).get()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Series' object has no attribute 'get'. Did you mean: 'ge'?

[rodrigogiraoserrao]

All of the nulls in this table are actually None, aren't they?
And true should be True and false should be False.

[rodrigogiraoserrao]

Suggested change

	| `implode` | `[ ]` |
	| `implode` | `[]` |

[rodrigogiraoserrao]

If you don't agree with my other subjective criticism on this paragraph, at least a couple of adjustments need to be made to fix typos and for consistency with the remainder of the docs:
(Again, one sentence / line would make it easier to review my suggested changes.)

Suggested change

Polars tries to follow aggregation semantics that match closely with [set theory](https://en.wikipedia.org/wiki/Empty_set#Operations_on_the_empty_set) and python semantics. This means that we might differ from SQL semantics for operations on operations on empty Series. For example, `pl.Series([], pl.Int32).sum()` is equal to `0` in Polars, where it would be a missing value or `NULL` when following SQL semantics. Consequently, `.group_by().agg()` on columns with `null` values might result in different results than would be given by an SQL engine. Below, we provide an overview of all aggregations and the return value when performed on an empty series.

Polars tries to follow aggregation semantics that match closely [set theory](https://en.wikipedia.org/wiki/Empty_set#Operations_on_the_empty_set) and Python semantics. This means that we might differ from SQL for operations on empty series. For example, `pl.Series([], dtype=pl.Int32).sum()` is equal to `0` in Polars, but it would be a missing value or `NULL` in SQL. Consequently, `.group_by().agg()` on columns with `null` values might result in different results than those that would be given by an SQL engine. Below, we provide an overview of all aggregations and the return value when performed on an empty series.

Or, “but it should be None if we followed SQL (semantics)”.

[rodrigogiraoserrao]

To build the docs just run mkdocs serve from the root of the repo.
What do you get when you run that command?

[coastalwhite]

Most of the python extension packages that we using are broken on NixOS. I have tried before to set it up and I kind of gave up.