This commit updates the horizon contributor docs to follow
what the community goal [1] recommended.

* Reorganize doc/source/contributor/contributing.rst to the format
  suggested in the community goal template.
* Cleanup obsolete information and topics covered by the general
  OpenStack contributor guide.
* Move contributing.rst to the top in TOC of the contributor guide
  as it is considered as the entry point for new contributors.
* Sync CONTRIBUTING.rst at the top level directory with the
  community goal template.
* Update the core reviewer expectation along with PTL expectation.

[1] https://governance.openstack.org/tc/goals/selected/ussuri/project-ptl-and-contrib-docs.html

Story: #2007236
Task: #38525

Change-Id: Idce7fc08e7fdf854d8a7bca55169d18403362c45