AVRO-3861: [Build] Add RAT exclusions for python docs

[sarutak]

AVRO-3861

What is the purpose of the change

This PR fixes an issue that the RAT check fails if Python API docs built are in lang/py/docs/build.
You can reproduce this issue by the following steps.

1. Build Python API doc by cd lang/py && ./build.sh doc
2. Run ./build.sh dist

Then, you can see the following error message.

[WARNING] Files with unapproved licenses:
  lang/py/docs/build/html/_static/pygments.css
  lang/py/docs/build/html/_static/basic.css
  lang/py/docs/build/html/_static/doctools.js
  lang/py/docs/build/html/_static/documentation_options.js
  lang/py/docs/build/html/_static/language_data.js
  lang/py/docs/build/html/_static/searchtools.js
  lang/py/docs/build/html/_static/sphinx_highlight.js
  lang/py/docs/build/html/_static/alabaster.css
  lang/py/docs/build/html/_static/custom.css
  lang/py/docs/build/html/automodule.html
  lang/py/docs/build/html/index.html
  lang/py/docs/build/html/intro.html
  lang/py/docs/build/html/genindex.html
  lang/py/docs/build/html/py-modindex.html
  lang/py/docs/build/html/search.html
  lang/py/docs/build/html/.buildinfo
  lang/py/docs/build/html/searchindex.js

To fix this issue, this PR addes an exclusion rule to pom.xml for RAT.

Verifying this change

Confirmed ./build.sh dist succeeded even if there are some files in lang/py/docs/build

Documentation

• Does this pull request introduce a new feature? (no)

[paliwalashish]

+1

[martin-g]

Thank you, @sarutak !