Improve contributing guide & docs

[bourgeoisor]

This PR:

• Completely revamps and cleans up the contributing guide
• Adds various templates for new samples
• Cleans up the root README
• Adds a template for new PRs

How to review?

• Take a look at the brand new contributing guide (https://github.com/GoogleCloudPlatform/kubernetes-engine-samples/blob/165155aa33ada94103c2d08ed924bd86254d724d/.github/CONTRIBUTING.md) and provide feedback (errors, room for improvement, unclear text). Note that some URLs are broken since they're pointing to files that are only checked in with this PR (chicken and egg issue).
• Take a look at the added templates and see if those would help when creating new samples (or if they make it more confusing, of if there's any room for improvement).

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• .github/CONTRIBUTING.md:39, tag gke_topic_sample_title_file_name

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[NimJay]

Suggestion:
I assume some contributors won't know what a "region tag" is.

Suggested change

	* [ ] Region tags have been properly added, if new samples.
	* [ ] [Region tags](https://github.com/GoogleCloudPlatform/kubernetes-engine-samples/discussions/878) have been properly added, if new samples.

Feel free to ignore this suggestion or use a different link.

[bourgeoisor]

Thought about it but I think it's best to keep all source of truth / docs / details in the "contributing.md" file (which the PR template points to). My answer to "what if someone doesn't know what a region tag is" would be "then they haven't read the contributing guide, which is the first checkbox they're supposed to check"

[NimJay]

This looks amazing! Thanks, @bourgeoisor!

Approved. Feel free to merge whenever — my comments a mostly minor issues.

[bourgeoisor]

Cleaned up the contributing guide some more.

Will merge as-is, but always open to improvement / tweaks even post-merge!