[docs] remove pmml redirect, simplify some other docs

[jameslamb]

I'm planning to invest heavily in the project's documentation for the next release. I think there are some opportunities to make things simpler and to make it more likely that people are able to find the information they need without turning to Stack Overflow or the issues pages.

This proposes the following:

• removing the pmml/ folder
  • it's just there to preserve a link to https://github.com/jpmml/jpmml-lightgbm, but that project is already linked to from the main README
• removes language about preferred style in the Python README
  • this is enforced via linters and doesn't need to also be documented in the README (which, remember, is also the homepage at https://pypi.org/project/lightgbm/)
• other small removals

[jameslamb]

This was added back in #2192, around when GitHub first added the Used By metric (#2192 (comment)) that you can view on the repo homepage.

I think having it here could be confused as a documentation of how to use the project, and that it should be removed to simplify the instructions a bit.

[jameslamb]

This is no longer true, as of #5837

[jameslamb]

Appveyor CI is failing across several PRs, like this:

  File "C:\Miniconda3-x64\lib\site-packages\conda\exceptions.py", line 1129, in __call__
    return func(*args, **kwargs)
...
  File "C:\Miniconda3-x64\lib\site-packages\urllib3\contrib\socks.py", line 75, in <module>
    class _TYPE_SOCKS_OPTIONS(typing.TypedDict):
AttributeError: module 'typing' has no attribute 'TypedDict'

(build link)

There WAS a new urllib3 release just 8 hours ago: https://pypi.org/project/urllib3/#history

BUT... the Appveyor jobs should be using only Python 3.8

LightGBM/.appveyor.yml

Lines 5 to 6 in 7d33879

	configuration:
	- '3.8'

so it's surprising to see Python 3.7 being pulled in:

    python-3.7.12              |h900ac77_100_cpython        18.0 MB  conda-forge
    python_abi-3.7             |          4_cp37m           7 KB  conda-forge

And Python 3.7 being pulled in is what's causing this error. typing.TypedDict was added in Python 3.8: https://peps.python.org/pep-0589/.

I suspect that what's happening here is:

• the conda that ships with the Appveyor images by default defaults to Python 3.7
• urllib3 is a dependency of conda itself
• this is failing at the point of running one of these updates, before ever creating an environment:

LightGBM/.ci/test_windows.ps1

Lines 47 to 49 in 7d33879

	conda install "brotlipy>=0.7"
	conda update -q -y conda

• the newest urllib3 version is not compatible with Python 3.7

We should probably update python in the base environment there at the same time we install other updates. I'll push some commits here trying that. Might as well just do it on this small docs PR.

[jameslamb]

looks like setting updating the Python version at the same time that we update conda worked 🎉

I'll merge this to unblock CI.