sort out contributor role with clear advantages, responsibilities

As per email conversation on geotools-devel:

* roles/contributor <-- renaming this page to be a role rather than action
* procedure/contribute <-- take the discussion and talk first, how to play nice with others stuff here
* procedure/contribution_license <-- how to fill in and email a contribution license
* procecure/pull_request <-- strictly focused on review of pull requests

This change only re-arranges existing content and does not effect any new policy.

Specifically Andrea has requested a change proposal to clarify the size of contribution requiring a contribution license. That is out of scope for this pull request.