Update join strategy in the user guide

[Sol-Hee]

Description

I think the document needs to be modified in the join part of the transform of the User Guide.

Although asof was deprecated from how parameter in join method by #2752, it is described as the join strategy in the document. So, I assume it would be confuse for user.
In addition, the introduction of Asof also states that you can join by using strategy="asof" in the join function, but it is deprecated, resulting in an error. (on v.0.20.10)

| ValueError: how must be one of {'inner', 'left', 'outer', 'semi', 'anti', 'cross'}, got asof

so, I would like to modify the document in the direction below.

---

1. Modifying the hierarchy

# as-is
|- Join strategies
    |_  Inner Join
    |_  Left Join
    |_ ......
    |_  Asof Join 

# to-be
|- Join strategies
    |_  Inner Join
    |_  Left Join
    |_ ......
    |_  Anti Join
|- Asof Join 

---

2. Modifying the Asof join description: Delete the following part
In Polars we can do an asof join with the join method and specifying strategy="asof"
Also, as shown in the image below, I would like to remove the asof, which is introduced as one of the strategy lists, so that users do not get confused.

---

3. Modifying the first part of the phrase
| Polars supports the following join strategies by specifying the strategy argument
In the Py-Polars, the strategy has been deprecated, so I recommend to modify docs as user should use the how argument.

If you have any better opinions, please feel free to let me know. : )

Link

No response