Remove unused front-matter from provider documentation

[modular-magician]

Description

This PR:

• removes the layout front-matter from documentation markdown files and template files
• removes the .erb template files that the layout value refers to
• removes the sidebar_current front-matter and template files linked to the sidebar navigation in the documentation.
• removes the .erb template file for the sidebar in the docs

The accepted front-matter values in the Registry documentation are page_title and subcategory. The description field is less clear - I've been told that it should be being used to set meta tags for SEO even though the documentation doesn't mention this, so best to leave them alone!

Other fields like layout and sidebar_current are not supported anymore - e.g. here's an issue in the aws provider that describes sidebar_current needing to be removed. By removing them it'll make it easier to debug documentation issues etc

Impact

Lots of markdown files have the 2 fields deleted from them in this PR

Templates and non-markdown files affected:

• mmv1/templates/terraform/resource.html.markdown.erb - stop it adding the 2 frontmatter fields
• mmv1/templates/terraform/resource_iam.html.markdown.erb - stop it adding the 2 frontmatter fields
• mmv1/third_party/terraform/website/google.erb.tmpl - delete it as it's unneeded
• mmv1/third_party/terraform/scripts/sidebar/sidebar.go - delete it as it's unneeded
• tpgtools - tpgtools/templates/resource.html.markdown.tmpl

This PR should not affect the appearance of documentation in the Registry providing that:

• All MD files for resources and datasources have subcategory still set
• All MD files in the guides folder have page_title still set

Open questions & stuff

• During this work I found this PR updating generation of sidebar_current in tpgtools - I believe that this could be removed, unless I've misunderstood something!

• Also I'm aware this change cannot be tested other than by making a release - sorry about that 😬

• Depending on how modular-magician works, I think this layout file in the downstream repo might need to be deleted via a PR directly against the downstream?:

  • website/google.erb

References

• Supported YAML frontmatter - https://www.terraform.io/registry/providers/docs#yaml-frontmatter
• PR in the aws provider where sidebar_current is removed.

Checklist

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).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• 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).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

Derived from GoogleCloudPlatform/magic-modules#6238