[SPARK-32507][DOCS][PYTHON] Add main page for PySpark documentation

[HyukjinKwon]

What changes were proposed in this pull request?

This PR proposes to write the main page of PySpark documentation. The base work is finished at #29188.

Why are the changes needed?

For better usability and readability in PySpark documentation.

Does this PR introduce any user-facing change?

Yes, it creates a new main page as below:

How was this patch tested?

Manually built the PySpark documentation.

cd python
make clean html

[SparkQA]

Test build #126889 has finished for PR 29320 at commit 86be1f5.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[viirya]

main package? main page? I mean the PR title.

[srowen]

Looks nice

[nchammas]

What's docs/img/pyspark-components.pptx for?

[HyukjinKwon]

What's docs/img/pyspark-components.pptx for?

It is for the image I used in the main page in case some people want to edit. There are other pptx files in docs/img as well for that purpose.

[viirya]

This looks good. I just found a few typos.

[SparkQA]

Test build #126951 has finished for PR 29320 at commit 6d5f6ef.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #127046 has finished for PR 29320 at commit dae09ec.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[HyukjinKwon]

Thank you @viirya for approaching this. I am merging this to master.

[BryanCutler]

Looks great, I think it will be very helpful for PySpark to have it's own main page. Thanks @HyukjinKwon !