Rework + reflow provider reference, touch up index

[modular-magician]

Use https://registry.terraform.io/tools/doc-preview to preview. Sorry, this ended up being a larger change than expected!

Remove the "Quick Reference" from the docsite, it was annoying to link to the full reference in practice because both items shared links. Instead, break it up by use cases: auth, quota, provider defaults, advanced (networking minutiae really).

Also clean up the endpoint and batching references. They were not up-to-date. For endpoints, we should add them to generated resource references directly out-of-band with this.

Argument reference appears on the landing page for AWS and Azure, add it to ours for consistency (but just link the preexisting page). Also update very old contribution instructions, link to discuss.hashicorp.com, and flip around the learn links + getting started link.

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Generated Terraform providers, and ran make test and make lint in the generated providers to ensure it passes unit and linter tests.
• Ran relevant acceptance tests using my own Google Cloud project and credentials (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
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Derived from GoogleCloudPlatform/magic-modules#7437