generate: Remove only tfplugindocs managed files and subdirectories from rendered website directory

[SBGoods]

Relates: hashicorp/terraform-provider-null#218

Previously, tfplugindocs would remove the rendered website directory and all subdirectories if they exist before generating new website documentation. This method prevents documentation not managed by tfplugindocs to coexist in the website directory. This change modifies the generation to only remove subdirectories and files directly managed by the tool including: "data-sources", "guides", "resources", and "index.md".

Due to the lack of existing end-to-end testing, this was verified by locally compiling and running against a branch of terraform-provider-null which had a cdktf subdirectory within docs. Running the command excludes the cdktf subdirectory from deletion while still generating documentation from the templates and examples as expected.

$ ./tfplugindocs generate
rendering website for provider "terraform-provider-null" (as "terraform-provider-null")
copying any existing content to tmp dir
exporting schema from Terraform
compiling provider "null"
using Terraform CLI binary from PATH if available, otherwise downloading latest Terraform CLI binary
running terraform init
getting provider schema
rendering missing docs
generating missing resource content
resource "null_resource" fallback template exists
generating template for "null_resource"
generating missing data source content
resource "null_data_source" fallback template exists
generating template for "null_data_source"
generating missing provider content
provider "terraform-provider-null" template exists, skipping
rendering static website
cleaning rendered website dir
removing directory: "data-sources"
removing file: "index.md"
removing directory: "resources"
rendering templated website to static markdown
rendering "data-sources/data_source.md.tmpl"
rendering "index.md.tmpl"
rendering "resources/resource.md.tmpl"

[bflad]

Hey @SBGoods 👋 Definitely 👍 on handling this somehow, but I'm a little curious about two mutually exclusive ideas here:

• Should cdktf be automatically excluded so folks do not need to discover or figure out the exclusion flag? Since CDKTF documentation is intended to be a "first class" provider documentation feature, I think it might be worth giving it special handling out of the box.
• Should the directory deletion be limited to "known" directories instead of deleting all other directories? I think this might overall a better behavior for the generator rather than assuming full control over the docs/ directory contents and requiring folks to figure out exclusion rules, etc.

[bflad]

Looks good to me 🚀