Add acceptance tests and ignore no such file or directory errors in cleaning rendered website dir

[SBGoods]

1. Adds testscript acceptance tests for tfplugindocs generate subcommand:

• null_provider_success: successful run of tfplugindocs on a stripped-down version of the Null provider with pre-exiting templates, examples, docs, and non-tfplugindocs managed docs (docs/cdktf subdirectory).
• framework_provider_success_no_templates: successful run of tfplugindocs on a Framework provider with examples but no templates or pre-exiting docs.
• framework_provider_success_named_templates: Successful run of tfplugindocs on a Framework provider with "named" template paths (i.e. templates/resources/.md.tmpl). Templates test all implemented data fields and functions.
• framework_provider_success_generic_templates: Successful run of tfplugindocs on a Framework provider with "generic" template paths (i.e. templates/resources.md.tmpl). Templates test all implemented data fields and functions.

2. Ignore no such file or directory errors in cleaning rendered website directory: the acceptance tests revealed a bug introduced in generate: Remove only tfplugindocs managed files and subdirectories from rendered website directory #267 where providers with no existing docs would return early with an error when deleting the rendered website directory before generation.

failing test output

[stdout]
        rendering website for provider "terraform-provider-scaffolding" (as "terraform-provider-scaffolding")
        exporting schema from Terraform
        compiling provider "scaffolding"
        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
        generating template for "scaffolding_example"
        generating missing data source content
        generating template for "scaffolding_example"
        generating missing provider content
        generating template for "terraform-provider-scaffolding"
        rendering static website
        cleaning rendered website dir
        [stderr]
        Error executing command: unable to generate website: open $WORK/docs: no such file or directory
        
        [exit status 1]
        FAIL: testdata/scripts/generate/framework_provider_success_no_templates.txtar:4: unexpected command failure
        
    --- FAIL: Test_GenerateAcceptanceTests/framework_provider_success_no_templates (21.02s)

This PR includes a fix to ignore that specific error and not return early.

3. Add unit tests to test resourceTemplate.Render and providerTemplate.Render

[bflad]

Please reach out with any questions!

[bflad]

After this should be good to go!

[bflad]

Looks good to me 🚀

[bflad]

❤️

[bflad]

❤️

[github-actions]

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