Organizational Templates lack documentation on how to use with GitLab

[jkodroff]

We released new GL features that allow orgs to use GL as a source of org templates: https://www.pulumi.com/blog/gitlab-better-than-ever/

In Organizational Templates we need to make the following changes:

1. Existing information for working with GitHub should be under its own heading
2. Add instructions for using GitLab as a source for organizational templates and note any limitations clearly in notes blocks of type info

[jkodroff]

@meagancojocar @dschaller Specifically, we have this sentence:

Pulumi Cloud now fully supports organizational templates stored in GitLab repositories.

But there's no mention of GitLab in the linked page, so I'm figuring we did have something written at some point and maybe forgot to merge it?