Rename contributions guide, update doc, PR template.

[robshakir]

 * (A) CONTRIBUTING.md
   - Add a top-level contributing document that links to the wider
     document discussion.
 * (R) doc/external-contributions-guide.md -> doc/contributions-guide.md
   - Rename contributions guide based on `openconfig/public` being the
     primary SOT for models.
 * (A) .github/pull_request_template.md
   - Establish a template for new PRs to the `openconfig/public` repo.

Change Scope

• Update documentation and artifacts within the OpenConfig public repo to reflect changes to the contribution process.
• Add a PR template (this one!) that helps ensure consistency in submissions and ensures standard questions are being considered.

Platform Implementations

Not applicable.

[OpenConfigBot]

Compatibility Report for commit dcba8622e43a7612a03410198af0dc3c3a7c3e26: ⛔ yanglint@SO 1.10.17

[robshakir]

@dplore - are we OK to merge this? It's currently blocked just based on a permissions nuance, see #708. I can skip the branch protection and/or ping Anees if we prefer.

[dplore]

@dplore - are we OK to merge this? It's currently blocked just based on a permissions nuance, see #708. I can skip the branch protection and/or ping Anees if we prefer.

I override the branch protection rule to merge. Thanks for sending the PR for updating branch protection.