[TASK] Add a code of conduct document

Having a CODE_OF_CONDUCT.md document that links to the TYPO3 code
of conduct provides users who view the project source on GitHub
easy, direct access to the full text.

Releases: main, 11.5, 10.4
Resolves: #97376
Change-Id: I6509b402a5229cbd926ce42b4fd6a1c0e60e0fc2
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74300


Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Lina Wolf <112@linawolf.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Lina Wolf <112@linawolf.de>