docs: Add default template that diverts to sub templates

[jbkyang-nvi]

What does the PR do?

This PR fixes this known github issue where multiple PR templates is not supported in Github.
It is an add on to PR 7290

Checklist

• PR title reflects the change and is of format <commit_type>: <Title>
• Changes are described in the pull request.
• Related issues are referenced.
• Populated github labels field
• Added test plan and verified test passes.
• Verified that the PR passes existing CI.
• Verified copyright is correct on all changed files.
• Added succinct git squash message before merging ref.
• All template sections are filled out.
• Optional: Additional screenshots for behavior/output changes with before/after.

Commit Type:

Check the conventional commit type
box here and add the label to the github PR.

• build
• ci
• docs
• feat
• fix
• perf
• refactor
• revert
• style
• test

Related PRs:

#7290

Where should the reviewer start?

.github/pull_request_template.md

Test plan:

I tested it with my own repository:
In the UI, if you create PR:

switching to Preview shows this:

You can click on one of the options:

to create the Pull Request.

If the PR requester skipped this step, they can populate the template themselves: https://raw.githubusercontent.com/triton-inference-server/server/main/.github/PULL_REQUEST_TEMPLATE/pull_request_template_external_contrib.md

Caveats:

This does not populate the correct template immediately, could cause confusion

Background

Multiple PR templates not supported in Github: https://stackoverflow.com/questions/73771068/multiple-templates-for-pull-requests-on-github/75030350#75030350
Alternatives are to use query parameters that are hard to use: https://github.blog/2018-01-25-multiple-issue-and-pull-request-templates/

Related Issues: (use one of the action keywords Closes / Fixes / Resolves / Relates to)

• closes GitHub issue: N/A