Update CONTRIBUTING.md

[grif-on]

Which branch your pull-request should merge into?

• 2.1: BugFixes and Improvements for Current LTS branch
• dev: New features or remove deprecated features

Describe your changes

Add missing C++ and Python code style links to CONTRIBUTING.md

Python code style link is taken from this old commit 32ba12c (this is the last commit that has code style links)

Issue ticket number and link

Checklist before requesting a review

For each PR

• Add Copyright if it missed:
  - "Copyright (c) 2019-present Axmol Engine contributors (see AUTHORS.md)."

• I have performed a self-review of my code.

  Optional:

  • I have checked readme and add important infos to this PR.
  • I have added/adapted some tests too.

For core/new feature PR

• I have checked readme and add important infos to this PR.
• I have added thorough tests.