docs: clarify plugins folder sys.modules registration behavior

[shashbha14]

The Modules Management documentation currently assumes that the dags, config, and plugins folders are equivalent and
all get added to the sys.path list. This is incorrect because the plugins folder also imports all .py files in this directory
and registers them in the sys.modules namespace under the filename without the directory structure. This means that if you
have a file named hdfs.py in the plugins/my_project directory, the file will end up as sys.modules["hdfs"], which can
cause name clashes with other packages, such as the PyPI hdfs package. For this-
Two warning sections have been added:

• In "Built-in PYTHONPATH entries" - describing the behavior of the sys.modules registration.
• In "Use unique top package name" - describing the depth at which this can occur and the importance of the top-level
  package name being unique.
  This Fixes Modules Management docs do not mention plugins folder's sys.modules registration behavior #63548

[shashbha14]

The Static checks failure and the modification to output_pr_auto-triage.txt are unrelated to this documentation-only PR. They appeared after the merge commit from main. Could a maintainer please advise or re-run?