feat: Replace existing archetypes with three new discrete types

[ADubhlaoich]

Proposed changes

NGINX documentation broadly follows the Diataxis model, and we have specific conventions around certain types of documents.

This commit pares the exisiting Hugo archetype down to three that are closely related: how-to, concept and tutorial. Outside of providing page-specific information architecture, these templates include our updated frontmatter formatting and parameters, and inline guidance on how we prefer to use Markdown formatting and Hugo shortcodes.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md and CHANGELOG.md)

[mjang]

As a next step, it might be worth working with https://github.com/hawkeyexl/doc-structure-lint, which can supposedly verify rules for each document type. (I tried it with an earlier version, but did not test Manny Silva's fix)

[ADubhlaoich]

Automation of template checks is a bit farther in the future as we still need to figure out details for other document types which have been inconsistent across different products, like release notes.

Gitlab has a few lightweight shell scripts for Markdown linting which could probably be turned into part of an action.